I'm standing outside this bonfire, I myself own a 08' GXP, there's a car here, with GXP badges on it. But no side vents, different bumper looking almost identical to a regular Grand Prix, but it also has the HUD found in GXP.
Did they make a GXP with no side vents and a regular bumper? Or is this guy a fraud?
Actually, I think I have seen some '05 pictures of the GXP proto that look like you describe. There may be a handful of 05 gxp's that have no side vents. It would be super duper rare if this were the case. Most likely an "upbadged" one. Only the GXP (even those prototype models I mentioned) have the silver dash trim around the speedo/tach. This should be a dead giveaway.
There was a local small time car dealership that had a gxp. Looked at here pictures online. Had correct motor, brakes, gauges. But the bumper and fender where standard grand prix.. I figured it was in an accident and they just replaced wit wrong parts.