The Grand Prix remained basically unchanged from 2004.
With the Base sedan, GT, GTP, and the new GXP.
The base model and GT had the naturally aspirated 3.8 L V6, with the GTP having the 3.8 L V6 with supercharger.
The Comp-G package was still available for the GTP with an added door emblem denoting the Comp-G package.
 When the GXP debuted later in the 2005 model year, the Competition Group was dropped, but several features were retained in a Sport package available on the GTP for the rest of that model year.

The GXP powerplant is the LS4 V8, a 5.3 L Displacement on Demand (active fuel management) engine based on the LS1 block.
Output of this version is 303 hp (226 kW)/323 lb·ft (438 N·m).
It has a shortened crankshaft, and a host of other modifications to make it fit into a front-wheel-drive vehicle.
The GXP also has a 4-speed automatic transmission with paddle-style TAPshift, heads-up display, vented cross drilled brakes with PBR calipers, performance tuned suspension with Bilstein gas-charged struts (sits about 9 mm (0.4 in) lower than other GP models), Magnasteer II, and StabiliTrak dynamic control system.
Also, the DIC will display G-Force (maximum achieved for lateral, acceleration, and deceleration) information when the car is not moving.
Cosmetically, the GXP differs from the other models with more aggressive bodywork including a different front clip, wheel well cooling vents on the front fenders, a different rear bumper, and twin-dual polished exhaust.
The Grand Prix GXP also offers a unique feature of the different-width 18 in (46 cm) polished Alcoa forged aluminum wheels.
The front wheels are 8 in (200 mm) wide, and the rear wheels are 7 in (180 mm) wide.
The Grand Prix remained basically unchanged from 2004. With the Base sedan, GT, GTP, and the new GXP. The base model and GT had the naturally aspirated 3.8 L V6, with the GTP having the 3.8 L V6 with supercharger. The Comp-G package was still available for the GTP with an added door emblem denoting the Comp-G package.  When the GXP debuted later in the 2005 model year, the Competition Group was dropped, but several features were retained in a Sport package available on the GTP for the rest of that model year.
The GXP powerplant is the LS4 V8, a 5.3 L Displacement on Demand (active fuel management) engine based on the LS1 block. Output of this version is 303 hp (226 kW)/323 lb·ft (438 N·m). It has a shortened crankshaft, and a host of other modifications to make it fit into a front-wheel-drive vehicle. The GXP also has a 4-speed automatic transmission with paddle-style TAPshift, heads-up display, vented cross drilled brakes with PBR calipers, performance tuned suspension with Bilstein gas-charged struts (sits about 9 mm (0.4 in) lower than other GP models), Magnasteer II, and StabiliTrak dynamic control system. Also, the DIC will display G-Force (maximum achieved for lateral, acceleration, and deceleration) information when the car is not moving. Cosmetically, the GXP differs from the other models with more aggressive bodywork including a different front clip, wheel well cooling vents on the front fenders, a different rear bumper, and twin-dual polished exhaust. The Grand Prix GXP also offers a unique feature of the different-width 18 in (46 cm) polished Alcoa forged aluminum wheels. The front wheels are 8 in (200 mm) wide, and the rear wheels are 7 in (180 mm) wide.
Pontiac Grand Prix