The Pontiac GTO and Holden Monaro are all based on the same coupe platform.
The Pontiac G8 and Holden Commodore are based on the same sedan platform.
Since all 4 have the same powertrain options, all 4 may very well be based on the same primary chassis/platform, with some secondary differences between the coupe and sedan entries. I think a Pontiac G8 would look awesome with GT5 graphics but that's just more wishful thinking.