Have your car taken to the dealer via flat bed. As for your potential problem, everybody's already hit on all of them...
The thermostat definitely sounds screwed up. It may be stuck closed -- or partially sticking. Either way, you're going to get water temperature fluctuations -- and the fans are going to be kicking on/off.
The water pump may be prematurely failing -- so you're getting inconsistent circulation. If, for some reason, the pump slows way down, the coolant in the engine is going to go way up. You never know, if might be the drive belt or something. Doubt this is the case.
While there has been a problem with the gauges sending wacky readings, such as pegging the coolant temp -- I just don't think that's the issue here. When this happens, the temp, speedo and everything else kind of freak out for a couple of seconds -- then it goes back to normal. And coolant definitely doesn't leak everywhere.
It could also be some kind of sensor that went on the fritz, too. You know with all the electronics cars have these days. Had a Mustang that started smoking like a damn bus driving home from work -- and it was an ECM that crapped out after something like a year.
The good news is that the car has excellent engine safety protection features built in -- so your engine, with very close to absolute certainty, is fine.
Any good tech is going to approach your problem in a methodical manner -- eliminating the usual suspects until something definitive is found. For anyone to leap straight to the conclusion that it's the gauge really isn't doing their job. Talk to a couple of service managers at nearby dealers first -- find out what their approach is -- then go to the one who sounds like they'll take their time and get this problem solved on the first trip.
Best of luck to you. Overall, the GTO has excellent build quality -- so once these bugs are worked out -- you'll be loving your new car.
Last edited by b_a_betterperson; 12-26-2005 at 07:21 PM.