In this guide I'll try and briefly touch on some of the things to get you started speedrunning Gunfire Reborn. (Emphasis on try and briefly because i find it hard summarizing things so this might be long)

#Overview
First of all the game is constantly changing since Gunfire reborn is still in early access - Weapons, scrolls and possibly characters are still yet to be released.
If you're looking to do your own research and be a bit pro-active you can always look at any of the most recent runs posted in the category you're trying to run - Generally if there's quite a few times posted already you can have an idea of what strategies are being used. Sometimes it's quite difficult to put into words the strategies used or the newer runner prefers to learn by watching instead of reading.
If there's no runs or very few runs in a category there is always the #Speedrun-Discussion channel in discord, to ask any questions or some sort of specific strategy you don't quite understand.
Once you've gotten an idea of how the run should go your best bet is to just jump right in and attempt it yourself - Execution in terms of runs has a very low ceiling to begin with so getting a reasonable time isn't as hard as you might think.

#Recording your runs and submitting them
Recording your runs locally is fairly simple - The rules have a fairly comprehensive list of things you should/should not do.
For software there's tons of free options such as OBS or StreamlabsOBS (Each are very similar) NVIDIA's Shadowplay (Provided you have an NVIDIA GPU) I've heard of people using Xbox Game Bar (I have 0 knowledge on the program but it seems to work for some runners).
Once you have your recording software you might want to trim your run down for a number of reasons - Removing resets or cutting out a long section after the run is over - I personally use Avidemux for my runs but it's not the most user friendly, OpenShot, ShotCut and DaVinciResolve are pretty decent free and more user-friendly editors. The software doesn't matter too much just find anything that works for you.
Submitting your runs via twitch or youtube - These two are going to be the main options for submitting.
If you are submitting runs from twitch make sure you are submitting a Highlighted version of the run and not a time-stamped past broadcast as these are not permanent and will be removed from twitch after a period.
Submitting from youtube is just as easy as uploading and providing a link - If your youtube video is limited (Not viewable in some countries due to copyrighted music) the run might be declined - You can use youtube's editor to remove the audio if that's the only option you have.
After all that make sure your runs don't have any elements covering the timer or any of the other vital elements for the speedrun.

PLEASE READ THE GAME RULES BEFORE ATTEMPTING RUNS, There are vital things we need to have shown in a run recording to verify, All runs MUST feature both Challenge Success screen (The screen shown after clicking the portal to end the run) and the Player Information screen (This is done by selecting it on the main menu after the run) It helps to click on the specific run you just completed to show scrolls etc. This is outlined very clearly as we have to reject any run without that info, and it really hurts to reject good runs without it.

#Speedrunning Tips
So probably what most of you want from this guide is some tips or thing to give you that edge in running I'll try and break this down into sections.

#Know your strategy
Have your main strategy for the run in mind and aim to hit that, there's quite a lot of RNG involved with replicating an ideal run for each strategy so don't be afraid to reset runs if it's not looking to give you the time you need - But don't be too over-critical, I've had runs i have been convinced will die and lose tons of times for them to completely turn around and save 30+ seconds in the later sections. Identify what things are critical for the success of a run and what things are "Nice" or could save time but not ultimately make or break the run - An example of this might be Foundry only Qing yen, In this run it is pretty vital you get Easy Kill early on to be able to stack damage and Max Cleave to enable cleaving consistently. Things that would be "Nice" in this run and save time are things like %Damage for cleave or AOE for cleave - Stuff that isn't "required" but you might want to fish for on that perfect run.
This is going to vary a ton depending on what time you are aiming for so judge and learn what it is you need.

#Tier-lists of ascensions
So this is something that could easily be conveyed and put into a simple to read "Do and don't" tier-list, but it's extremely important to understand people play the game differently, have different skill levels or just prefer to avoid something we consider to be "Meta". What i recommend every runner is to have their own mental tier-list when it comes to ascensions so you can make decisions on what is best for a run but still keep an open mind to try new things.
There are ofcourse clear wrong choices in runs like for example choosing some other level 1 explosive ascension when you have the option of Mad bomber 3, but for other things the line can be blurred and rng can speak for itself.

#Room Variations
This one is something that is not very obvious to new runners and something you pick-up and learn later on, Each level in the run has specific variations - These rooms can hugely influence the time of your run some rooms wasting upwards of 45 seconds in just waiting around for mob spawns (At the worst). There's no clear indication which rooms save and lose time this is something that has to be learned - Possibly something the community can look into and map out which rooms are the best.
Each room has a set group of options, What i mean by this is 1-3 for instance can have upto 5 different options each with their differences in their time to clear.

#Vaults
Generally in an ideal run you shouldn't have to take any vault but to start out you might want to consider taking them for a few fairly quick scrolls, Learn which vaults are fast and what take far too long, if you enter a vault and it's going to take too long just simply leave - The speedrun being in-game time checking vaults wastes only a few seconds. In Elite and Nightmare even top runners opt to take a couple of quick vaults.

#Red chests (Cursed chests)
These are extremely valuable in a speedrun being able to offer you cheap and sometimes even free scrolls/weapons/health, Generally in runs the option for sacrificing health for Scroll is very good - the bun drops being fairly plentiful.
Be careful about choosing damaged or dusty scrolls, each of the options has a different % chance of the rarity of the outcome, The lower the tier the higher chance for Cursed scrolls.
Upgrading normal and replacing all for weapon mods, This is something that isn't greatly explained, Upgrading normal will take any random "Green" inscription on a weapon turning it into a "Blue" mod, Sometimes this isn't a direct upgrade as some blue mods can have things such as "Damage/But -Magazine size" so be careful and pay attention to the possible outcomes. The best scenario for this kind of upgrade is if you have say a Justice with -50% AOE and all the other mods are Blue/Orange you can remove the -50% AOE resulting in a much more usable weapon.
Replacing All is strong for taking a poorly rolled weapon but a weapon you want to use, Small things about the replace all - It will only keep the exact same amount of mods it has on before so replacing all on a 1 mod item will still keep it 1 mod, It will re roll the item as if it has dropped new - So it's possible to get higher rarity mods on it, If the weapon doesn't have a Gemini mod on it - And you've upgraded the weapon to at least +5 when you replace all it will take one mod at random and turn it into a gemini. (And if it has one mod only it will become a weapon with just Gemini and no inscriptions).

#Mob Spawns
If you've played the game even just a small amount you may have noticed the enemy spawns are really not random - It's possible to learn and map out almost all of the spawns in any given room, This is extremely important when it comes to saving time especially in the larger rooms being able to perfectly map out your route/order of clearing the room for being optimal. This is something you cannot simply teach, my tip if you're still learning is to always be looking at the minimap in down-time.
A smaller thing to learn is in some rooms mobs spawn in sequentially and this is not limited to wave-respawn rooms, There's always a specific order it, it would be hard to give examples in text but you'll notice it once you start paying attention to them - Things like "Late-spawns" where an enemy will only spawn after you've moved to a certain point in the room or cleared X mob or all mobs. There's quite a few of those in some of the 1-2 and 1-4 Variations if you want to look out for them.

#Boss strategies
It's quite hard to put into a list some of the choices for weapons/scrolls for killing bosses that would take an entire other guide, but i will list some small tips for each boss.
Lu wu - Most if not all of his abilities can be dodged simply with a reactionary dash and strafing
Golem - Standing slightly off center to the right can make it a little bit easier to hit his crit spot while his arm is up, Piercing weapons can actually go through his arms.
Icthy-something i forget the name - In the head phase the boss will tilt its head just before doing the beam, Dash immediately in the opposite direction and begin strafing (Make sure you're not extremely far away or too close to cause the melee attack), While the boss is burrowing you can actually hit it as if normal and even the tail is possible to be hit if moving away from you. During the tail phase it's really simple to time a grenade/cleave/kinetic orb to destroy all the corrosive projectiles as they are fired. If the boss enters the tail phase with somewhere around 80-90% health (Not armour) the slam becomes a triple slam instead of one. This boss is different as their health isn't all red (Meaning fire weapons being less effective against the first bar) If possible it's highly recommended to have an alternate decent weapon for melting off the armour if you're not Crown prince, Something like a porcupine or a decent corrosive weapon, But ultimately if you have enough damage the armour-type health won't mean much. The way in which the boss dives is somewhat random and "Can" be manipulated but would lose time anyways. Yoruhume - Generally this boss is just aim to burn him down as quick as possible there's not a huge amount of strategy.
In the cannon phase it can help to start killing the cannons and slowly move forward sometimes avoiding the projectiles by going under them. The octopus spawns are entirely random from what i can tell so just check the minimap or just do a quick check both sides. Placing the cannon ball is done by interacting with the tiny little platform on the back of the cannon.
In the second phase his crit spot is the eyes, You can shoot down the projectiles the cannon fires with one hit, and if you're using justice you can aim up and time the detonations to blow up the cannon shots while also dealing optimal damage, Making the fight very safe.

#Boss Quick kills
This is something that can save you huge amount of time in runs, Since 80% of the run is about clearing mobs quick and efficiently your weapons and ascensions will likely be catered towards that playstyle leaving bosses to be fairly slow and sluggish, All of the bosses in the game can be quick killed (With the exception of the final boss) - This being the equivilent of a "one-cycle" for other games. Strategies as simple as grabbing an off-hand weapon specifically meant for killing an upcoming boss, For example a crimson firescale is my favorite weapon to use for quick kills on Lu wu/Golem and prove to be very effective - Simply hit as many crit shots as you possibly can while spamming firebomb as soon as it's off cooldown, you want to be fairly close to hit your shots easier. There are countless strategies and specific combos that can aid in quick-killing a boss and couldn't possibly be specifically listed.

#General movement and Secondary usage
Most weapons in the game have a "Weight" to them slowing your character's base move speed, Things like the tiger cannon, big hippo are very heavy weapons and Submachine guns are light barely slowing you down - Use this to your advantage while moving through long stretches of rooms, Ultimately you can press 3 and swap to the foundry it being the lightest weapon making you move faster.
Of course you want to be dashing at every available time as you progress from A to B clearing a room.
Position yourself closer to the door while the Golden goblet chest opens, you can interact with it from quite a distance away - Very minor but could save a quite a bit over the course of a run.
Secondaries (Grenades and cleave) do not interrupt reloading at any point, use this to your advantage - However Primary abilities will interrupt reloads.

#Challenge rooms and Elite spawns
Generally challenge rooms and elite spawns are completely random, Each of them being quite nice for the run if they don't lose you time, When clearing a challenge room - The last enemy to die in the room will drop either A Bun/Weapon/Scroll so be sure to check where that last enemy died. In some cases the drop can be pulled to a specific location in the room if it's out of bounds. Same with Elites but they can drop A Bun/Weapon/Scroll/Golden Goblet, the difference with elites it's possible to actually roll all 4 of them at once dropping all of the rewards, in most cases dropping 1 or 2 of the possible rewards.

#Taking your time and making decisions This section is just about abusing the in-game time rule by using things like Peddler/Ascensions/Pausing to take your time on decisions and consider your options since the in-game timer is paused. This can also be used while the in-game timer is paused after killing a boss in co-op to exchange scrolls consider what you want/need for the next stages.
A lot of running this game is about improving your decision making in terms of how to clear rooms, when to use abilities what weapons to take, if the somewhat under-powered weapon is good because of X inscription, Taking X ascension to enable a strategy. There's countless things that you can consider when making decisions in the game, and you'll see runners with a lot of experience be able to make the decisions much easier so they can focus on gameplay optimization. This is really something you can only learn by either playing a ton and learning combinations/options or by watching/talking with other runners, i highly doubt a runner would be opposed to answering a question of why they use X thing and to explain their thought process.

#Improving your runs
If you're at a point where you don't see obvious ways you can improve your run or you're stuck on what to work on or progress, I'd suggest asking in the discord to maybe have someone go over and give some constructive criticism to help improve, I personally (Pimmalage) would love going over runs and maybe making videos reviewing runs to help that specific runner and hopefully other people who watch to improve.
I'd like to touch on a point of not everyone is going to get those golden times instantly, a lot of the runners have been playing for a long time and 100's of attempts at a single category to get that "one" perfect run. Spend less time comparing yourself to their times and focus on your own PB progression as it's something a lot of people i think can get caught up in.

#Closing thoughts
Of course i cannot possibly cover every single thing that goes into a run but hopefully this is enough for some of the newer runners to get off the ground and into some great runs.
I know for a fact i skip a ton of the weapon choices and scroll choices for each category, but each of those could be an entire guide in itself, so i skipped out on some of the "Meta" things the game currently has. If that's something that a lot of newer players really need help with i can look into making something along those lines, but would take a ton of effort.
If there's anything you specifically want me to add into the guide that you want to know more about or have wrote yourself i'd be glad to add it in as a collaborative effort.