You can buy it in the shop. That is not only the easiest way to get gold, but you also support us to further develop the game.
Alternatively, you can earn free gold by watching ads directly in the HUB. The offers to watch ads for free gold will appear once there are any ads available. If your gold rewards were not paid out straight away, try restarting the game. You should get a small check with the gold.
Finally, you can take up Free gold offers under the Free gold section.These offers present various ways how to earn more gold in the game.