... I didn't answer you because I haven't test it thoroughly yet. I did manage to change Siren's L Mag-RF to ST Mag-RF, so it looks promising.
in 'main', look for kernel.bin. G-Force table is in section 3, offset x0F78 to x17B7, lines are 132 bytes long. Among with other stuff, their base abilities (minus 'steal') are listed in there.
EDIT : by editing this table you can only change the abilities a GF can learn, not the ones they know already prior to joining your party. For example, Siren is supposed to have Mag-J already learned when you get her, and that ability is listed in the table. But if you edit the table and replace it with a Spr-J, the only thing that will change is that she will be able to learn Spr-J with APs, Mag-J will still be here & already learned.
I suspect there is as well level&prior ability requirements in there, and that the order in witch they are listed is what the game will follow for learning them if you don't chose manually. (For example, SumMag+10% comes first)