Engine will only start when I put my foot on the gas pedal?
This would lead me to believe that you have a vacuum leak. This may be an external leak or internal. The most common would be a cracked PCV valve hose or a sticking IAC Valve. The Idle Air Control Valve is what controls the vehicles Idle so that it maintains a nice and smooth 800 rpm(in neutral). Replace the IAC if you suspect it is the root of the problem.
IAC Valve 2002 Pontiac Grand Prix
I went ahead and looked up the IAC Valve that will fit your 3.1 liter engine in the Grand Prix. Not too expensive and only requires the removal of two screws and one electrical plug connector.