Gold is the official in-game currency in Atlantica Online.
Gold is kept in your inventory (you can see how much you have at the bottom left of the screen) and in the bank.
Gold in your inventory is needed for any in-game transactions, such as purchasing items from the market.
You should keep extra gold in your bank. If your Main Character dies without an active Blessing License, you will lose 10% of the gold you have on you. Gold in your bank is safe from this.