|Darksol the Painbringer|
Optimization makes static bonuses the better thing to specialize in. Even if Damage Dice might be cooler and more thematic, Guts' character build wouldn't have the tools needed to really kick it off. You don't have size increasing abilities to make it shine, both virtual and actual. That's spellcaster territory; more specifically, Druids and Monks.
Large-sized Bastard Sword is the same thing as a Full Blade in 3.X. Unfortunately, -2 size penalty and needing to spend a feat to even use it at all is an issue to make effective, even with Bonus Feats. Greatsword would be a safer choice, but again, static bonuses are best, which means critical-focused weapons are better. This means 18-20/X2 multipliers with decent damage dice. The only weapon that does this is the Nodachi, which is just a giant ass sword with a big hilt; but, if your GM is willing to gestalt, I'm sure he wouldn't mind a simple weapon reflavoring.
Speaking of gestalt, while I'm not 100% sure of the rules, I suggest you pick up a different class besides Fighter. Sure, their situation improved, but honestly, Slayers are better, and do fit Guts' character better. You'll still get access to bonus feats, and in fact, can be used to better supplement Guts' other combat utilities (his grenades, Repeating Hand Crossbow, Arm Cannon, etc). There's also the matter that Guts is a cunning and devious combatant, especially when he's desperate, and has been known to use distractions to catch an enemy flat-footed (which in turn makes them open to a devastating Sneak Attack).
Race would be Human. You could go Dual Talent for the attribute increases, as you'll have plenty of skill ranks per level (4 + at the minimum), but the bonus feat isn't anything to scoff at. However, you should have plenty of feats, both bonus and otherwise, to make him able to function with most (if not all) of his weapons. The arm cannon is a gray area, and really functions as a McGuffin, because it's basically a Siege weapon shrunken down into a single mechanical forearm. I'd talk to your GM about how he would run it as, and plan accordingly. If it's a Firearm, you have it covered. If it's a Siege Weapon, well...that's more complicated.
Favored Class Bonus should go into the Human Barbarian Racial FCB.
Anywho, Basic Feat Progression:
1. Power Attack [duh]
2. Precise Shot [For the Repeating Crossbow]
3. Raging Vitality
4. Exotic Weapon Proficiency (Repeating Hand Crossbow)
5. Improved Initiative
6. Crossbow Mastery
7. Accomplished Sneak Attacker
8. Exotic Weapon Proficiency (Firearms) or Weapon Focus (Nodachi*)
9. Extra Rage Power (Lesser Beast Totem)
10. Point Blank Master
Rage Power Progression (basically cookie cutter):
4. Reckless Abandon
6. Witch Hunter
8. Spell Sunder
9. Lesser Beast Totem
10. Beast Totem
12. Greater Beast Totem
14. Guarded Stance
16. Rolling Dodge
18. Reflexive Dodge
20. Auspicious Mark?
Strength 20 (18 + 2)
Constitution 16 (14 + 2)
Skills (6 X 20 = 120 Skill Points)
Climb + Swim MAX (Let's just hope he never does so again in full armor)
That should about do it. Good luck, my friend.