There's booty grab and zOMG. Vending requires capital to start out with, so generally speaking, it's not the way to go for newbies to earn gold.
She asked how to earn Gold WITH Gaia Cash.
Technically speaking, you don't really need GC to earn gold on Booty Grab, if you're just going to other people's tanks.
That's why I said vending.
Well in that case, I'd actually say hoarding is the best way to go. I mean you could buy a bunch of RIGs and sell them off for an instant 90-100K profit, but IMO, you'd be better off hoarding them until they go up significantly in price. Alternatively, you could vend the items off and use the capital gained to vend some more, but that would require a good amount of knowledge of the marketplace. You could also buy a bunch of high-yield aquarium fish to stick in your tank to make gold off of booty grab. Alternatively, you could buy high gold:GC ratio items, vend those off, and use the capital to invest in high-yield aquarium fish. Vending is the fastest way though, but to get the most "bang" for your buck, alternative methods would be preferred.