I am taking a bit of a stab here, and don't have evidence I can back it with, but here goes. The "GTP" was supposedly Grand Touring Performance. Based on that, I would guess that it would be Grand eXtreme Performance. If anybody else has better evidence or info, I'd love to hear it.
I think I saw a link on another board that revealed this; when asked, a Pontiac executive admitted that the designator "GXP" did not have a specific meaning. It was just a marketing device used to designate a trim/performance package from others in the same line. I don't remember exactly where the link went as it has been some time since I saw it, but it was to an entity that you would think would be reliable (ie Motor Trend). Look at it this way, everyone knows "GT" stands for "Grand Touring" (or Turismo) and "SS" stands for "Super Sport", if "GXP" meant something, I think the definitive word would be out by now.