There are several different options available for getting Gold. You can purchase it using real money to buy Diamonds which is the premium currency and then trade it for Gold. This is done by going to the 'Top Up' tab in the Store and purchasing one of the six options available to you. The exchange rate is roughly 1 Diamond equals 10 Gold.
If you do not want to spend real money to get Gold you can earn it by completing Daily Challenges. You can also recieve Gold from Supply Crates which you may initially have to spend Gold to open if you do not have the required amount of Supply Tickets. The best way though of getting Gold is through the Daily Missions and the easiest way is to regularly log into the game.