Gold is a special currency that is obtained through either buying it with real money, or getting a small amount of gold received from the Daily Reward. In-game gold can be used to buy just about everything.
Things you can buy with gold range from: Veteran membership, Soldiers, Soldier's uniform skins, Assault Teams, Vehicles, Vehicle Supply Crates, Weapons, Weapon Modifications, Weapon skins, WarBonds, Ribbon boosters.