So I finally found a place that would tune the 2005 GTP. I asked if they tuned a 2005 Grand Prix GTP, and it doesn't sound like they ever have. Below is what they said:
"Yes we do tune all kinds of cars.
It appears that we can tune the 2005 Pontiac Grand Prix GTP (Supercharged 3800) with our software.
Does it have any additional modifications done or is it stock and you just want it tuned up ??
The cost is $125 to license the vehicle with the software and then it is $210 for the 1st hr and $125 per hour after that. Average vehicle is on the dyno 4-5 hours, depends on how much tuning needs to be done.
You would need to drop the vehicle off for a couple of days and schedule it in. "
So would this be someone you would take your GP to, to dyno tune?