Excess gas is used before a gas booster!
So I had 300+ gas, and used the free gas booster. At the end of it I realised my gas had gone down. Sent a support ticket.
The response is that you will always use excess gas before a gas booster.
That is absolutely stupid. If i have free gas, I should be using that free gas.
and thanks for contacting us!
Full Gas will fill your gas meter to the maximum capacity for the 24 hours once you chose to apply it, and will always replenish your gas meter to your current maximum capacity (shown in the details of your Mission Car in the camp).
Any excess gas you obtained over your maximum capacity will always be used first when going on missions and will never refill, not by a Full Gas booster or by buying a Gas Refill.
Thank you for your feedback; we will make sure to clarify this information when applying a Full Gas booster in a future update.