Gold coins are added by Tale Of Kingdoms and used for buying and upgrading parts of the kingdom. They are gathered in 3 ways.
Gold coins can be gathered a few ways. One, used passively and yeilds the most coins over time, is killing mobs. The mobs can be any kind including mobs from other mods. Killing any hostile mob gives the player 15 gold coins. The other, with a small amount over time, is doing quests. This not only gives gold, but also a boost in other stats within the TOK mod. The last way is to sell items to the the kingdom, this is done by locating sell boxes in the kingdom. They are found in places like the blacksmith, or the bakery.
Gold coins are used for a number of things. The main use is to upgrade your kingdom by adding more buildings and people. The other use of the coins is to buy items from your built kingdom. You can buy items such as diamonds, armour, and weapons. There is an UNCONFIRMED myth that gold coins can be used to buy items from other mods, such as Divine RPG.