This achievement is fairly straight forward. It may seem like a lot but as the enemies get more difficult, the Gill rewards get higher.
Tips to earn this Quicker:
-Don't waste gil on stuff you don't need. There are plenty of items obtainable via chests. Choose your jobs based on whats available. I find that weapons are pretty easy to come by. Good armour is easy to come by later in the game. Once you get to Saronia, you shouldnt have to buy armour anymore. You will find what you need in chests.
-Sell your old\weaker items.
-Dont buy Potions. Collect the ones you find and make sure you have a job in use that can at least use lvl 1 Cure.
-Don't Flee from enemies. Fighting all enemies is great way to improve your job level, gain experience and earn Gil.