actually,gau has 256 rages.there's rage #00 (hexadecimal),so when you count from zero to 255,you get 256 (zero is first,one is second,two is third...255 is 256th) rages.in the menu,the last rage is left blank,but it is actually the enemy "pugs" (no,not the enemy "pug").it cannot be selected as an available rage.of the 255 left,3 are unaccessible:ziegfried,chupon and allo ver.so that means the available UNCHEATED rages for gau is only 252.

and to answer your question,i have completed all 252.don't ask how long it took.^_^

EDIT:The enemies in ffvi does exceed 255.There are enemies without rages (e.g. proto armor).of the 8 dragons,only the white dragon is in the veldt,and is therefore,the only one with a rage.and though you cannot have kefka's rage,you can have the flare star ability through the io rage (not sure).i am sure though,that there is a rage whose ability is flare star nd i have used it.so coooool...