I had the same problem too when I first got my G8. Gas door flew off during the air dry and put a small chip in the paint. The problem comes when you pull into the car wash and put the car in park, it automatically unlocks the doors (including the gas door).
So all you need to do is after you put it in park, simply lock the doors again.
So all you need to do is after you put it in park, simply lock the doors again.