It depends how much Gil you need and what you’re willing to do to get it.

Personally, I prefer a couple quick things every week to bring in some cash and then some things to avoid expenses. The game gives you a lot of tomestones of Poetics. With those, I regularly buy stacks of demicrystals at Idylshire. These cost 25 poetics and sell for 500 Gil to a vendor. It’s very easy to get 2000 poetics every few days, so that’s a regular source of money. Not big money, but constant money.

I also have my retainers bringing me those 500-Gil Allagan coins.

For avoiding expenses, you also get allied seals and grand company seals that you can spend on teleport tickets. 5 allied seals for 1 teleport ticket can save you 999 Gil. Your grand company seals can buy a teleport ticket that takes you back to your grand company HQ, again saving you up to 999 Gil.

It all adds up and takes very little time. For lots more Gil, you have to invest more time. Sell your high end materia, find and sell high value craft materials and things like that.