Here are two great places to collect massive amounts of Gil and experience. On the game's very first chapter, DG Soldiers will respawn over and over again in certain areas (specifically around the gates). Kill them without going through the gates. You can earn seemingly infinite amounts of Gil and experience here.

The other great place to attempt Gil and experience farming is in the chapter entitled "Deepground Strikes Back". Before entering the HQ entrance, DG Soldiers will be stationed outside. They will respawn infinitely, making this area another place to earn seemingly infinite amounts of Gil and experience.