Gold is the currency of Thalmont, and The Hero can obtain this currency by doing many things, including selling items, completing Quests and looting corpses. Gold is used to buy items, learn new Skills, or employ the services of a Skill Changer. Gold can also be used to bribe people in certain quests.
Some quests reward gold for completing them. On the quest pages, it is given as "x# Gold." What this means is that if you times your current level against that number, you will get the number of gold you will recieve for completing the quest. For example, "x30 Gold" on a level 20 character would result in 600 gold (20 * 30 = 600) and "x80 Gold" on a level 50 character would result in 4000 gold (50 * 80 = 4000).
You can add gold to your inventory using the console. See the Cheats page.