Most likely that is a wiring issue - something isn't connected to the right thing.

The G6 uses a brake pedal position sensor, not a simple brake switch. If you replace the switch or the body controller or if the brake lights stay on you may need to calibrate the sensor using a scan tool. You probably need to take your car to a dealer. From the service manual.... The brake pedal position sensor (BPPS) receives a low reference signal and a 5-volt reference signal from the BCM. Whenever the brake pedal is applied, the BPPS applies a variable voltage signal to the BCM through the stop lamp switch signal circuit. The BPPS calibration must be performed after the BPPS or body control module (BCM) have been serviced. The calibration procedure will set BPPS home value. This value is used by the BCM to determine the action of the driver applying the brake system and to provide this information to the vehicle subsystems via the class 2 communication bus. Calibration Procedure Install a scan tool. Select Diagnostics. Turn ON the ignition, with the engine OFF. Select the vehicle. Select Body and Accessories. Select Lighting Systems. Select Module Setup. Select BCM. Select the BPPS calibration procedure and follow the directions displayed on the screen.