Vending and hoarding are always fun.

Gaia games like fishing, blackjack, towns, slots, and all of those don't make you a ton of gold, but they're still fun.

Booty grab can make you a shitton if you have time for it, it's probably the best legitimate way.

Gaia Cash is great if you have the money, normally $10 = 1m Gaia Gold when you do the gaia cash method. It's a bad price if you were buying gold on the underground, but since it's legit and can't get you banned, it can be a good price.

Besides that, if you want illegitimate:

There's botting, which can be profitable but it's easy to get banned for.

Phishing, which can get you banned easily too. It's what FLPs and FMPs are, when you're stealing a person's login credentials. Normally if you're doing this, you just sell the accounts instead of risking your main getting banned.