There is plenty to spend gil on. Not everyone crafts, so alot of people still require gil for materia, low-level gear, food and consumables, etc. If you do craft, alot of folks still require gil to buy certain mats they themselves may not be able to get, also, repairs at 50 take their toll.
For example, I have about 50k and after buying food for coil runs and repairs (sometimes in excess of 1k), I find my 50k now dwindling to 45k, and I expect that will eventually fall to 40k and so on. I have gathering classes but since I'm ALWAYS undercut I cant seem to make enough money to reconcile what I must spend, so it is becoming a vicious cycle with no gain.