It means "more expensive, with go fast stuff added."
It most likely means nothing - just "GXP"
They had GT, but wanted something more special...
So they did the Trans Am GTA.
Had the Pontiac Grand Prix GT, but to bump that up, they didn't want to use the GTA, so they called it GTP.
Now they want something "cooler and faster sounding" than GTP - so they come up with GXP. Makes it sound exciting, like some
Xtreme
Xperimental version of a GTP.
Oooh, G
XP.
What do you want to bet GZP is next? Oh wait, Chevy had an LTZ, so they might not want to use the Z like that. Maybe the GYP? That's got to be it.