I'm currently making a 100% guide. Should Gadwin 1 be part of 100% completion? He is beatable, but Justin needs to get to 250 strength and have midair cut. This would require many hours of grinding at such an early point in the game. 

I'm asking the mods. Should he be part of 100% completion?

In general, I'd say it's up to the community as a whole. With the extreme grind required, I personally don't feel it should be required (since you're not intended to defeat him in a regular playthrough, and to me, 100% is a regular playthrough, just obtaining everything that's available to obtain). So unless you get something special by defeating him the first time that can't be obtained in any other way, I don't think it should be required.

I actually think you messaged me about it last month as well on Twitch, and I sent you a response on there as well. But yea, I personally don't feel it should be required, unless there's a reward that can't be acquired any other way.

The problem with that is there isn't much of a community here (at least not yet). And I just looked up the boss fight and you do not win anything from him, You get 0 XP, 0 gold and no items. The reason I had asked this was because I've seen a few RPG 100% speedruns requiring to defeat every boss, no matter the difficulty (such as Alpha Weltall from Xenogears).

In my opinion, however. I don't feel he should be included either. He isn't beatable when you normally play through the game and I don't think he was ever meant to be. But this, of course,  this is not my decision. 

Such abilities like Time Gate, which also requires extreme grind, should be included. Because, even though there was a code error before the release of Grandia, I still feel that this should be obtained as 100% completion. Yes, the run will be VERY long with this requirement, but the game isn't 100% without it. But yet again, this isn't my decision. I'm not a mod, YET...... ;)

Time Gate I definitely agree on should be part of it (as it's part of the game, and something that was intend for you to be able to obtain). The same goes for the optional bosses in FF games, some of which can be ludicrously difficult, but they are designed to be that way, while still being possible to beat. Where as Gadwin isn't supposed to be beaten (at least not at the point when you first encounter him).

If I remember correctly, however, there's some optional dungeon in the game, but I can't remember what's in it.

Yes, there are 3 optional dungeons that have bosses and items in them that cannot be obtained anywhere else. 

There is: 
-Soldier's Graveyard has the Kung-Fu Master (looks like Master Chang), 
-Castle of Dreams has Lord's Ghost (looks like Mage King) 
-Tower of Temptation has Leviathan (looks like Squid King), Gargoyle (looks like Ganymede) and Slug Fish (looks like Grinwhale). 

All these bosses I think should be included as part of 100% completion and they are all the only optional bosses in the entire game.

Gadwin 1 absolutely should not be a requirement of 100%. He's not 'meant' to be beatable. 

Here are Gadwin 1's stats:

Str: 999
Vit: 999
Wit: 45
Agi: 30

It's absolutely ridiculous to expect a player to fight against this. He's BARELY beatable in ReDux, and that's with higher player stats, his VIT reduced to 245, and greater access to debuffing attacks like Def-Loss having a greater effect. It's not uncommon for other games to forgo a specific objective in 100% simply because that objective is completely unreasonable, and this is Grandia's equivalent. I'm personally not even that happy about including Time Gate as a requirement as it is a pure grind that takes hours and adds nothing of value to the run, but it IS an obtainable spell, so I suppose it's technically arbitrary to claim it shouldn't be included just because it takes ages to do.

It's safe to say that Gadwin 1 will not be included as part of 100% completion. He doesn't drop anything relevant for 100%. Drako, Kargha, myself and other people who have expressed interest in running the game all agreed upon it. I'll lock this thread for archival purposes, since this thread is currently 8 months old, so no need to keep it open. Cheers.