The official website for The Settlers of Catan

Question about General Questions Resources - What can I use gold for in the Introductory Game?

Answer

Gold is only used for trading or for choosing cards you want to add to your hand. The Gold Ship allows you to trade gold for another resource at a 2:1 rate. In addition, the Goldsmith and the event Traveling Merchant enable you to trade gold 3:2 and 1:1, respectively. In the Theme Games, particularly in The Era of Gold, gold is more important.