Gold coins (called "golds" and abbreviated "G" in the games) are the minted currency of most of the countries of Pangaea. The currency seems to be interchangeable between countries and even continents, as Hare uses it on both Bubalook Island, Aurora Island, and the Northern Continent.
You can accumulate golds in the games by winning tournaments (most battles offer a cash prize for winning), selling items or monsters that you trained, or through some in-game events. You must use your golds to pay for food each week, buy training gadgets and upgrade your barn, and buy needed items. Some cities charge a fee for combining monsters at the Studio as well.