The only GT's that were factory supercharged were the 06 and 07 GT's because Pontiac dropped the GTP nomenclature from the Grand Prix line up. If your car is a 04 GT and is supercharged, someone swapped it. If you scroll through your DIC there would be a factory "boost gauge" if your car is indeed factory supercharged.

As it stands, its basically pointless to have a boost gauge in a supercharged application anyway.