Page 8-499 from Volumn 2 of the G6 service manual says:
1. Remove the license plate lamp screws.
2. Reach into the opening in the rear bumper fascia.
Rotate the license lamp forward and downward.
3. Pull lamp through the opening.
4. Turn the license lamp socket counterclockwise to release the socket from the lamp housing.
5. Remove the bulb, if required.