no GTP is the superchagred one. the two door one is just a grand prix coupe. either supercharged or not. gt or GTP. ill let 99gtx tell ya what the GTX package gets ya.
oh boy i prefer not to completely explain it since iv done it too many times since i got the car... here ya go
C/P
http://www.grandprix.net/limited/gtx_article.html
SLP's Grand Prix GTX (Production)
What do you get when you add ram air induction to an already power-ridden Grand Prix? SLP's GTX, that's what. The basic package includes ram air induction, special GTX badging to replace your standard badging, and custom GTX key fobs. Ram air adds approximately 10 horsepower to your Grand Prix, be it an SE, GT, or GTP. Other items available include a full stainless steel cat-back exhaust system, sunroof, synthetic fluids package, and more.
NOTE: This is not a limited edition because SLP will make as many as possible.
Special Features
Standard Features ($2499)
Fully functional ram air hood and revised intake system (adds ten horsepower)
Cloissonne GTX badging for the doors and trunk lid
Custom GTX key fobs
Full 3 year / 36,000 mile warranty coverage provided (Saved my ***...****** went out)
Federally crash tested and proven design
Optional Features
Dual/Dual exhaust with four outlets (Adds five horsepower, $699)
17" x 7" Six-spoke chrome-plated aluminum wheels and Michelin P235/55/R17 Pilot series tires ($1,999)
Custom front and rear floormats ($119)
GTX ram air custom car cover ($159)
Visor-mounted programmable garage door opener and Home Link Security system ($199)
Webasto X-48 SS Spoiler type power sunroof with sunshade ($799)
Synthetic lubricants package, engine oil and filter ($59)