Gold is the premium currency of the game and is normally purchased using real money, it is used to buy the premium planes or buy supply crates. However there are several ways that you can get gold for free.
Daily Login
Between 100-200 gold can be earned free as a reward by logging in daily 15 times during each month (doesn't have to be in a row)
Contributor
You can earn gold for free by getting the staff of Wings of War to approve you as a contributor (make videos) or help out as a moderator.
Special Offers
Occassionaly the game will offer promotional events that will give you free gold.
Game Friends
Adding a friend to your friends list in the game will net you 50 gold and a blueprint.
Event Center
If you tap on the 'Fan Page Like Event' in the event center, you’ll be taken to the War Wings Facebook page. Liking the page gets you 100 gold.
Linking your War Wings account to a Facebook account will net you 100 gold and a blueprint.