Things that create gil:
- Rewards from quests, levequests, FATEs, and duty roulettes
- Autoselling items to NPCs
- Gil dropped by enemies, especially in dungeons
- Challenge log rewards
- Housing auto-demolition refunds

Things that remove gil:
- Teleportation fees
- Market board tax
- Buying items from NPCs
- Repair fees
- Purchase of housing plots

The activities in the first set tend to create more gil than is removed by the second set, for someone who does not interact with the player economy. The rest is simply redistribution of that extra gil in exchange for goods and services.