Earning gold is simple. Almost all actions that you take in Gaia will earn you gold: playing a game (including zOMG!), surfing the maps, leaving profile comments, and even reading and replying to threads. You can also earn gold by setting up your own shop and selling services or items that you find throughout the world of Gaia.
In zOMG!, gold is dropped from all monsters and is a reward for many quests. It can also be found in chests.