You can always get gold by buying it with real money at the shop. However, you can earn some free gold, along with free stuff such as silver, experience, matierals, supplies, and equipment by completing the various Achievements and missions that you are given throughout the game. You can see what 'Daily Missions' and 'Career Missions' are currently available and the rewards they give by going to the 'Missions' menu. Also, don't forget the free daily login bonuses and the goodies these containers offer.
Campaign Mode 'Destroy Artillery' Mission Gameplay