In 2004 there was GT1, GT2, GTP, & GTP Comp G. GT1 & GT2 were naturally aspirated (no supercharger). The GTP & Comp-G had the supercharged V6 with the Comp-G getting some upgrades in suspension and some other goodies like HUD.
For 2005 there was GT, GTP, & GXP. GTP was NA, GTP was the supercharged 3.8, and GXP had the 5.3L V8.
In 2006 GTP was discontinued and GT gained the supercharged V6. The base Grand Prix had the NA v6. It pretty much remained this way through 2008 when the GT was dropped along with the supercharger.
