how else do you expect to get gil?? the only other way to really get gil is to pick it up randomly in chests (and you'll usually find this isnt very much)...

one of the fastest ways to get gil is by battling mimics in the omega ruins. they are found quite often by stealing from the chest in the battle with a Zaurus. you will get 50 000 gil from the battle, and if you use the gillionaire ability, 100 000 gil... thats pretty good if you ask me.

also, the monster arena bosses (especially original creations) drop some really sweet weapons, that can sell for nice amounts (ultima buster drops some weapons worth 200 000 gil each!!). however if you dont want to train to beat these monsters, just stick to fighting mimics.

hope that helps