Have a look through your challenge log and see what targets you want to go for. A lot of them have gil rewards, and additionally you get a 10,000-gil bonus for every five targets you tick off, so there might be some non-combat challenges you can do quickly to reach extra targets. (Eg. go to the Gold Saucer, play a few minigames and you can tick off two challenges - which might mean an extra 10,000 gil bonus for a few minutes' work.

Also just keep going with the story and quests! They give you a good amount of reward money.