Rage, when you get on the Veldt, Gau can leave the party for a short amount of time to learn monster skills. In battle he can use them by selecting a monsters rage. When he chooses, he goes into a thing comparable to berserk, where he can execute any skill the monster he's raging could use. Magic, Lores, normal attacks, etc. You can't control him at all during this. So it's just a bunch of luck.