Thought of an idea for making Gau less grind-y: Allow him to learn rages if you leave him on the Veldt.

Example:
He leaps into a monster group. You leave the Veldt and do whatever. Based on how long you've gone without him (probably based AP, similar to Terra's Trance), when you get him back, he'll have learned a certain number of rages (of the learnable ones by that point in the game), with some randomness factored in.