I'm not 100% on all the differences, but I know the exhaust is different, the rims are different, the interior has different options. I know mine has heads up driver seat full elec + elec lumbar and heated driver and passenger seats, not sure if the GT1 has it or not. The 'GT1' doesn't have any GT tag on the back of the car, it just has Grand Prix on the left of the trunk. The GT2 just has GT on the bottom right of the trunk. I'm sure there are more, but both are rated same HP (200HP). I guess it's possible the GT2 has a different handling package but I'm not sure on that. I can tell you though I thoroughly enjoy the crap out of my car, going from a pos '93 saturn to this, well words cannot describe.
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Grand Prix GT2 2004
ZZP Z7 Turbo Kit 62-1, BC, 42.5# Injectors, LT1 MAF, 160 tstat, A/A IC, BOV, 3" ZZP SS Exhaust /w cutout, GMPP Handling kit
339.2 WHP 358.6 WTQ @ 13 PSI (zzp's mustang dyno)