There is no doubt the gtp has the power necessary to tow. It's the weak fourth, and lack of a frame that keep it from having a tow rating. I would say keep it out fourth, mainly because of the non hardened splines, and unless your on a long flat straight you'll have a hard time keeping it from jumping between 3rd and 4th(you can't hold it in 4th without modifying a different trans table or something, which is hard on the transmission.
Monitor your trans temp, and also your boost and try not to sustain boost for long.
There's a hitch available here:
Curt 12225 - Pontiac Grand Prix Trailer Hitch 1997-2008 - Receiver Tow Hitches
also google hidden hitch grand prix and you'll find another.
