GMC 2007 Acadia Owner's Manual page 391

The tethered fuel cap is located behind a hinged
fuel door on the driver's side of the vehicle.
To open the fuel door, push the rearward center
edge in and release. The door will pop open.
To remove the fuel cap, turn it slowly
counterclockwise. The fuel cap has a spring
in it; if the cap is released too soon, it will
spring back to the right.
While refueling, hang the tethered fuel cap
from the hook on the fuel door.
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