There's gold in them there hills! Come on everybody, let's Run to the Hills and get some GOLD!
That may not have been as funny in text as I had hoped, but I think you get the point. You can get gold pretty much everywhere in this game. Small amounts come from every mission you play, larger amounts come from skull quests, and even larger amounts can come from precious souls, events and Gauntlet.
Gold is used for too many things to even mention. You can buy items in the store with gold, it's needed for upgrading characters and talismans, and you even need gold to open more souls. The last one is largely forgotten until you've opened enough souls to run out of gold. You're going to need plenty of gold througout the game, so keep collecting it by playing boss levels at the highest difficulty you can muster.