Very limited experience, but I'll toss an opinion on the pile...
Buddy owns a 4cyl auto G6. 0-60, bet it would keep up with my stock SVT. 40-80, think I would own it. 80+, bye-bye. All opinions courtesy of my self-calibrated butt. :laugh:
A GTP?? think I wouldn't be placing any money on myself, especially the early ones with 3900 & 6 speed manual. The later ones went with the 3.6L and auto only. Probably as quick off the line but not sure higher up...slush box but better power.