There's 4 ways to make decent to big amounts of gil in this game.

1. Put vendor crafting materials on the action house for gil. Highest profit margin, but requires a lot of micro management.
2. Crafting leve turn in items (HQ) and selling them to crafters who are leveling.
3. Crafting leve turn in items for yourself (HQ) and turning them in for gil profit (usually requires you to be able to gather most of the mats).
4. Level Goldsmith and Miner, and make ilvl48-49 electrum accessories for free. Then run dungeons and such with them, spirit binding into materia to sell. This is what I do, and I do ok on Balmung.