My favorite way to get GP was chocobo racing. It's a hassle, but if you feed your chocobo the right greens, it's not a problem to beat the ones in C or even B class. Just choose to recieve GP instead of a prize. ^_^ Eventually, you'll be all set to go duke it out in the Battle Square. Oh--and make sure you have plenty of curative items, in case your materia get disabled. O_o