To anyone struggling to have more than 1-2 mil at a time:

- Two level 80 battle retainers can bring 15-20k Gil / hour when you pick the right material. Ventures are by far the best use of GC seals assuming you don't already have too many venture coins. Even better if you have more than two retainers. (1 min each time, 40k+ Gil / day)
- Leveling roulette every day as adventurer in need. Buy crafting mats with uncapped tomes from the tome vendor. Try other roulettes for more Gil. (15 min, 30k Gil / day)
- Gather a zonureskin map every day. (2 min, 25k Gil /day)
- 6 cookie levequests per day. You can collect a map while you get coffee beans. (20 min, 50-100k per day depending on NQ/HQ)
- Dwarf beast tribe dailies, buy 3 of the rare crafting mat. (5 min, 20k Gil / day)

Check prices and make your own plan. It takes minimal effort to make at least 100k per day at level 80. You'll have at least 20 mil by the time Endwalker launches.