You might be overestimating the ease of finding info on Gatsu. You might want to tell us about the character in your own words. It will also help to tell us what sources you can use and what level the PC will be starting as (IOW, does your DM have a "death penalty" for new PCs).
I got this from the description of a game based on the Manga:
Because Guts' sword is so devastatingly huge, it can't be used accurately in close confines (except when in Berserk Mode). Luckily, Guts has his trusty hand crossbow to use when he can't wield Dragon Slayer. Also at his disposal are throwing knives (unlimited ammo), grenades, and a devastating hand cannon. Grenades and rounds for the hand canon can be found by destroying certain objects in the environment.
Near as I can tell, he's a guy with a big sword, a hand crossbow, throwing knives and the like.
The sword sounds unusually large- to me, that means its a weapon that is oversized as compared to the PC's size. In 3.5, there are few ways to do this: either you take the Monkey Grip feat or you must be one of the 2 races that have Powerful Build (Half-Giant or Goliath). You should note that PB is mechanically superior, but that MG is open to any PC...and that the effects of the 2 don't stack.
In addition, there are exotic blades out there that may have that extra-nifty to them that you might enjoy, like the Jovar...but that will require taking the Feat Exotic Weapon Proficiency: Jovar.
The hand crossbow is an exotic weapon, and that will require taking the Feat Exotic Weapon: Hand X-bow.
Knives are a simple weapon and cheap, but if you want to make them unlimited like the game, that will take some work. It can be done, no question, but its a tad exotic.
Grenades are usually simulated in D&D by Alchemist's Fire grenades. There are other grenades in the RW (and maybe the game, I don't know), and similarly, there are other alchemical grenades available.
The Hand Cannon would require either some kind of setting involving gunpowder- ask your DM- or some kind of exotic PC building that simulates the effect of having such a weapon.
Possible character classes include:
- Fighter
- Samurai (Oriental Adventures)
- Paladin
- Barbarian
- Cleric (certain builds)
- Duskblade
- Psychic Warrior
- Battle Sorcerer
- Kensai (PrCl)
Possible Feats include:
- Power Attack
- Cleave
- Great Cleave
- Monkey Grip
- Exotic Weapon Proficiency
- Weapon Focus
- Weapon Specialization