Gold is a special currency that is obtained through either buying it with real money, or getting a small amount of gold when you finish your first battle of the day. In-game gold can be used to buy just about everything.
Certain items like: Warbonds and Membership can only be bought with gold.
Things you can buy with gold range from: Membership, Soldiers, Soldier's uniforms, Assault teams, Vehicles, Vehicle supply boxes, Weapons, Weapon Modifications, Weapon skins, War bonds, Ribbon boosters.
You earn 4 gold upon completely your first match every day.