The description mentioned on this page matches this car:
http://www.pontiacserver.com/gph3_9.html
It is reported that a couple of "brass hat" cars were maroon, although the only one the author has seen was a prototype with a wild rear deck spoiler replacing the bubble roof.
The spoiler is the same style as T/A spoilers of the same vintage:
...but as mentioned, I'd want to see some paper work before shelling out any cash for such a car. I personally think the burgandy with beige interior looks great and honestly do believe it is what he says it is, but again... with a car like this, it's all about the paperwork, otherwise it's just a trailer queen GP you couldn't drive if you wanted. I'd like to see the VIN/serial number stamping as well - I don't know much about factory prototypes, but if that matches what typically happens with such cars, you're one step closer to authenticity.
The white paint in the trunk doesn't surprise me if GM was just trying out something different (keep in mind if the entire trunk is body coloured, it never got splatter painted, which means it could have been yanked from the line), but the original wheels should have tagged along with the car...