Your Pontiac's wastegate solenoid is a small black valve located near the wastegate actuator. It is attached to the airbox. Touble code P0243 indicates a Turbo / Supercharger Wastegate Solenoid A malfunction. Your Pontiac Grand Prix's wastegate normally runs a very low boost pressure. The Wastegate solenoid bleeds the manifold pressure signal back to the airbox to increase the boost pressure. If the engine control computer notices a wastegate solenoid fault it would de-activate the solenoid which would drop the turbo boost pressure.
We recommend using a hand vacuum pump to open the wastgate. If the hand pump opens the wastegate then you can move on to inspecting the vacuum line from the intake manifold to the solenoid. If vacuum is present all the way to the solenoid, and the solenoid is receiving a signal from the ECU to open under load, then chances are the solenoid is defective.
