Late in the game, visit Nam Cobanda Isle in the northwest part of the map. Go to the gym.
1. I bought 1000 chips (roughly 300,000 gald).
2. Save outside.
3. Play poker and bet the maximum 500 and try to win with anything above one pair.
4. Once you win, it will ask you whether you want to double up. Say yes. If it is a high card, bet low, and if it is a low card, bet high. For 7, 8, or 9, I usually just end it and take my winnings. (Doubling up a few times can mean a lot of chips.)
5a. If you run out of chips, load your previous save and try again.
5b. After winning a large amount of chips, go outside and save so you can just reload.
In the beginning, it will seem hard because you can only play twice before reloading. Once you have won a couple of games, you can bet the max a few times in a row before having to reload.
Extra Gald Tip: The healing items (special, miracle, treats) don't cost that many chips, but can be sold for gald to recoup your initial investment. Special Gels can be sold for 5000 gald each (15x5k=75k). For lots of gald, win lots of chips, buy all the gels, sell the gels (Yumanju's the closest town), and repeat.