You can try your luck at the crappy mini-games in wonder square, but the easiest way is to just buy them. There is a small chance that there will be a man near the ropeway car (to the right of the save point) who will sell you GP at an exorbitant price. He's hard to see, but he sells 1 GP for 100 Gil. Just find whichever game you're good at and try to rack up the GP there. If you're playing on an emulator the Quick Save feature is very useful on the Super Dunk game.