What rage(s) did you used with Gau (if you used him, of course)?
I mainly sticked with the Io rage, but I also tried some others, just to see. I remember using the Brachiosaurus, Behemoth, Stray Cat(because they say it is the best for him, but I didn't really thought so), and some others, but again, it was just to see what would he possibly do, without much interest in them other than seeing.
I really liked the Io rage from the first time I used it. Gau dealt as much damage as any of my other characters with a normal attack, but what made me really keep using it was the Flare Star, because Kuja's bias (Kuja was the first character I ever saw using Flare Star, and just like with Ultima, it became a trademark of him, I think).
The good part is that Io is a boss?(I think it is a boss) that you kill in World of Balance, relatively early if I remember well. So I manage to have a strong Gau relatively early! I'll leave the Io wiki page for anyone to check, as I think it is only called that in my version (or it has some other names at least).