Level 59 having trouble making gil.

[Legion88]

You can just farm dungeons for the Gil, but this means you need time.
Selling synth materials on the market board can also be a "fast" way, you just need to watch, what sells for how much and how fast.
You can obtain this materials via GC seals, tomestones and treasure maps.
Do your Challenge Log, it also rewards you with Gil.
Minions from Dungeons and some Orchestrion rolls are also selling for some good Gil.

The only way to get "fast" Gil involves crafting or gathering in the current level range.