First off, welcome to the site.
That's a common problem with our Grand Prixs. Mine does it fairly frequently. Your ignition isn't registering that the car is in park.
I tried to find a thread to link you to, but I can't find one.
I know there is plenty of them though. I'll keep looking, but I'm sure someone else can find one and get it to you.
*EDIT* AHA!
Temp. Fix Diagnostic/Repair