To open the gas tank on a 2008 Toyota Highlander, press the release switch under the dashboard.
There is an interior button that releases the fuel door on this Highlander. Look beneath the dash on the driver’s side for a black switch with a gas tank icon. Press it once, then exit the vehicle and go around to the fuel door. It will be slightly open, and you can open it completely and unscrew the gas cap.
If this button doesn’t work for some reason, there is an emergency release for the fuel door in the luggage compartment.