Gold coins are one of the currencies used throughout the game. Gold is received through many sources, and is the most easily acquired resource in the game.
Sources[]
Mazes, exploration (dungeon), exploration (campaign map), guild battles, gauntlet, trials.
The only action (which I can think of) not providing gold are the Chambers in the secret region.
Uses[]
The primary use for gold is purchasing cards from the altar, (5,000 for a single card, 22,500 for 5 cards). Read more about Coin Bag
The other, more noticeable use, of gold is card evolution / upgrading, which can be very expensive.
Furthermore gold can be spent at the Bazaar to buy Energy, Essence, Glory Cards, Hero Shards, and Creatures.