When your player level increases in the game you receive free fuel as part of your reward. This fuel is stored for you to use at a later time.
Whenever you select Refill Gas you will see Gas Storage at the top of the popup, the number under this heading is the amount of gas units you have in storage.
If you have gas available in your storage you will automatically use this gas to fill your vehicles.
'Gas needed' shows how many gas units are required to fill your vehicle.
If your gas storage is empty you can pay for gas with coins.