New players may find it convenient, and legal, to buy the Steam or GOG "versions" of the games and use them to speedrun. However, after a few runs they may wonder why their times are not competitive with the other runs on the board. This is because the Steam and GOG "versions" use an emulator named "ScummVM" as a means to play the game on a modern computer. If you were to download the files for the game and run the sierra.exe executable in the folder you will get a prompt from Windows stating "This app can't run on your PC" meaning that dos based applications do not run on a 64-bit PC natively. Unless you have an old 32-bit PC knocking about, then an emulator is necessary, but why is ScummVM not suitable for running King's Quest VI?
Here are a few points of interest:
First of all I would like to address something: you may notice that I am putting "version" in quotation marks when I refer to GOG and Steam. This is because GOG and Steam are distribution platforms selling digital "distributions" of these games. These distributions come bundled with an emulator that has been configured to work on modern PCs, as according to them. These are not "versions". Versions of Dos games refer to revisions of the engine, game's code, and localizations done by Sierra. Here is a link to a wiki page listing all the different SCI (or Sierra Creative Interpreter) engine games and their respective versions: https://wiki.scummvm.org/index.php/Sierra_Game_Versions#SCI_Games
A: ScummVM artifically throttles, or sets the cpu it is emulating to a certain mHz, which makes it fine for a casual experience, but not for speedrunning. Why is that? Certain tricks, such as the zips require a certain speed to be fully utilized. At slower speed you will see Alexander's legs moving very fast, and that is good, but it isn't as fast as he could be. Using DosBox ECE set to max cycles and dynamic core the zips are nearly instantaneous, which is what you would want with a speedrun.
B: While somewhat unrelated to the GOG or Steam releases of the game, our customized for speedrunning version of ScummVM, dubbed "Quan's ScummVM" as it was customized by community member UrQuan, exhibits a bug on faster CPUs when climbing the cliffs of logic. With the "Enable speed throttle" option in ScummVM disabled, holding enter when climbing the cliffs of logic on even a 4th gen intel processor will cause Alexander to constantly stumble and give the all too familiar "Woah wait a minute" voice line. Thus, we do not use this version as it only works on slower CPUs.

For these reasons, and more, we in the KQ6 speedrunning community prefer to use DosBox ECE (Enhanced Community Edition) for running the game. We have specific configurations to the .conf file to maximize speed and ensure as much stability as possible during speedruns. If you wish to buy the game legally and use it for speedrunning, then a little setup is required. Fortunately, there is a guide posted under "Guides" on this page to help get you setup doing so.

In conclusion, the GOG and Steam "versions" confusion is something that is always going to be present when talking about these games, but hopefully this post helped mitigate it. Speedrunning these old dos games is not quite as "plug-n-play" as console games, but with a little knowledge and light education it can be done easily and without many problems. Personally, I wish that we could have one emulator that was PnP to run every King's Quest game, and ScummVM would be my choice for that as it is open sourced to anyone wanting to make their own version, but we currently do not have someone in the community with the knowledge and experience to do so.
Thank you for taking the time to read this.