The bulk of my gil was made from desynthesis and gathering.

I also don't spend much gil because I craft all my own gear (for the most part, either my sister or I make gears for our characters depending on who has more free time) and do my own gathering. It's not a get rich quick method, but my gil pretty much only increases over time.

I typically "over-gather" when I do go out to get materials for crafting too. One, so I can have extra mats in case I screw something up (I do manual crafting) and two, so I have extra materials to sell after I'm done making my gear. I rarely go out just to gather materials for the purpose of selling, I simply sell off my leftovers.

I also send my retainers out on quick ventures and they find some surprisingly expensive things from time to time.

I typically do the "grindy" content when it's realeased. For example, Palace of the Dead, Heaven on High, Diadem, Eureka, etc. The drops from these places can sell for quite a bit the first week too, before the markets get saturated. I also did this with Rathalos actually... I cleared EX Rathalos around 80 times... so I ended up with a ton of normal scales leftover. In the ballpark of 280. Too goof off, I bought 10 well-done steak indoor furnishings and dropped them in my friends house... which still left me with 180 normal scales. So I bought 9 of each of the Orchestrion Scrolls and sold those off on the MB. I severely slashed the price because I wanted to move them... but sold them all in 2 days and made about 2mil from it. IMO, even my slashed price was overpriced considering how easy it is to get normal scales... but that's why I wanted to move them quickly before the masses figured that out.

Lastly, I try to keep my desynthesis capped... and since I don't like spending money that means the lazy way is to desynth gear from current tomes and dungeon drops. Which usually drop current crafting materials when taken apart. If I don't have a specific use for the material myself, then that's another thing to sell... but on my server sometimes the mats can be selling between 50k-100k each, so it's free gil for me.