Pontiac Repair: Engine Cooling systems, gasket kit, head gasket

Question
I have a 96 Pontiac Grand am, and am having problems with the cooling system. The thermostat is working properly, no overheating, there is no leaking from any hoses or connections, and the radiator resevior cap is replaced and on properly. Although after awhile of the vehicle running, well 3 1/2 weeks since I had to replace the heater core I havn't had any problems. Drove the car about 1500 miles in that time frame, check the coolant regularly. The vehicle wasn't overheating at all although as soon as I put it in park last night it started pouring coolant from the resevoir (over a gallon was lost). I have rechecked all connections and everything is still good. I cann't see any signs of the coolant in the oil from a cracked head gasket, or corrosion around the heads from coolant leaking out. The most I can come up with is that there is a internal leak. Am I on the correct path? What tests can I do at my home to test to see if that is the problem? And if it is, will I need just a new head gasket, or the entire head gasket kit? I have NO money to take the vehicle to a mechanic, only hopefully enough to pay for all the parts. Tearing the engine apart to replace the gaskets wont be a problem for me to do if absolutely necessary, I really hate tearing apart the engine and/or transmission. I've had to do it too many times with my dad, uncles, and brothers that I would hate to have to do it again and find out that it was perfectly fine. Please help, I just want to be done with all the cooling system problems with the car. Just a reminder there is NO fluid leaking from the radiator, NOR is the car overheating, it's just pouring the coolant everywhere.

Answer
Hi Heather I'll assume this is a V6? I'll also assume that the raditor cap is a factory cap and not a cheap after market one?  Usually head gaskest on these don't just do bad out of the blue They can but not that often. It's usually a progression from a leak and the engine is continued to run low on coolant to the point where is over heats over and over again.  A coolant pressure tester would be in order here to see if the sytem leaks down pressure but with the overflow tank on that engine you need a special adaptor or you need to pinch off the over flow pipe on the tank in order to pressure test it.  That may reveal a internal leak and then again maybe not.. I have seen head haskets go bad on the 3.1/3.4 liter V6 not leak pressure with a tester but the system will pressurize from a leaking head gasket. It's off and stange. I'll also assume that the electric cooling fans are working on this car? ? if you run the A/C after a minute or so at least one of the fans should come on that is if the A?C still works on a 16 year old GM car not all that likley... But it's a quick check of the cooling fan(S)
Good luck :)