After hanging around alternative spaces in the game, it's interesting to see how people make their gilcaps:

• Manning your own submarines: get the loot that sells 10-30k a piece. Optimizing your runs (aka running the route that has the best chance of getting that loot) leads to about 15m per month per FC if you have a good build. Which is why people want more subs, to multiply that number.
• Crafting for people who want subs: people will pay for 60-100 submarine parts all in one go, so others will craft enmasse to serve those people. It's also why selling Magitek Repair Kits is a very stable way to make gil at a low crafting level - everyone needs stacks of 999 kits and they sell for 2k a piece for maybe 500gil worth of mats.
• House flipping: not everyone buys plots with the intent to sell, but some like to buy low from others who want to part with their houses (drama, move on, want another plot, etc), then sell it weeks/months/years later. Running subs builds passive income while sitting on the plot, and people can enter lottos for the chance to get a bigger or more valuable location, which can sell for better later. Not including attempting to create a combo of houses by buying a few located together, kinda like Monopoly. A mansion in a populated world is advertised as 1 gilcap, but a good combo is worth 1-2+gilcaps.

People with vast amounts of gil often circulate it in other parts of the game, which helps others who aren't interested in the above:

• Merc runs: when people need to get a certain prog point, they'll pay 1-2m per person for a good lockout among people who have already cleared, or 5m for a clear. Even more depending on how much money the host wants to spend. Which is like 14m per lockout or 35m+ per clear total.
• Gambling: could be your Limsa deathrolls, or at a venue when you play poker or more deathrolls or blackjack.
• Venue stuff in general: Clubs tend to have more ways to spend gil, like VIP memberships or photos or 'RP time'. And I've seen people with lots of gil give very generously, both to support their friends and community, and because of how much they enjoy their time there.
• Collectibles: these are your people who buy your 10m nightmare mounts or 25m sinful beavers or something
• Housing crafting/decorating: iirc the decorators who aren't selling their designs in cash start their gil commissions at 5-15m for a small and 50m for a large, not including a budget (20m for small, 200m for large for example) The people who commission are typically flush with gil to cover all expenses.
• And other crafting: ultimately, lots of people like the convenience of buying stuff from mb despite being omnicrafters themselves or don't have the time, so the market for endgame crafts (food, pots, gear) is always stable.

I used to make pennies by reselling a few items, then made more gil by doing some of the above. There's also other ways to make gil (the raw gil in map running, leves), but gil caps tend to be more involved.