The GXP sensors are Wheel Speed Sensors. They provide the PCM the revolutions per minute of each tire. If a tire starts to deflate the revolutions count will change and this sets off the tire pressure warning. It'* very accurate and can go off if you change or rotates tires. With the car rolling down the road, sure of the tire pressures, hit the reset and it will "measure" the readings and set that as the norm. Alot better system really. Nothing to wear out. It also uses the readings for StabilTrack, AntilockBrakes, TractionControl, and TorqueManagment, and used by the TransmissionControlModule.
Owners in the past have had a few reported problems with the connections at the wheel getting wet, loose, and corroded. I put some dielectric grease in mine because I always disconnect the fronts when I'm out cruising. Disconnecting either of the front connections lets you "boil" the tires and improves light to light performance by turning off TractionControl and reducing TorqueManangement. Any tire slippage while the connections are intact will pull your timing and clamp fuel injection (GXP, not sure about gas manipulation on the other trim models)