Assuming you're a 4th generation owner ('93-'02) because of the gasket issue. Please include your year in future posts. If I'm wrong about the generation of your car, ignore what follows...
The temp guage is pretty much meaningless. It's controled by a "dry" sensor bolted to the head. That sensor and the gauge on the dash are for informational purposes only and have nothing to do with anything. The real temperature sensor is "wet," as in it touches coolant. The wet sensor goes to the computer, and the computer controls the cooling system. Unless you plug into the computer and read the coolant temperature sensors live data, you have no real idea of what the engines temperature really is.
Using a little common sense, clearly you're not overheating if the engine isn't running. It takes several minutes for a cold engine to even reach the temperature where the thermostat opens and begins letting coolant flow, then, if there was no coolant, several more minutes before temperatures approach "overheating".
So, there is clearly a problem with the sensor in the head, or the wiring between the sensor and gauge. Grab the Factory Service Manual and use it to help you diagnose the sensor. You will need a multi-meter, also called a volt-ohm meter if you don't have one. They are available locally as low as $6. Basically, you're going to probe the sensor for the value it is sending out and compare that to a table of temperatures. A cold engine is "ambient", so if its 86F outside the number reported by the sensor should cross-reference on the chart to 86F or close to it. If the value cross-references higher or lower, bad sender. If the value is what it should be, I'd suspect wiring.
If oil pressure is dropping too, this could very well be a wiring issue. Its possible that both guages have a common ground or a powered wire is bleeding or shorting to ground. A lot of these guages use resistance to report values. If the wiring is at fault, like a corroded ground wire, resistance becomes "infinite". Depending on the guage, this condition could result in a zero reading on one guage and an extremely high reading on another.
Here is a link to the Factory Service Manual. It will have diagnostic flowcharts to walk you through the diagnostic process step-by-step as well as providing the values you're expected to see. I'm thinking this is a grounding issue.
Some resources: These books are in .pdf format, in multiple parts. There is a free-ware pdf utility program there if you need one, which can be used to merge the sections into original size. F-PLATFORM (FIREBIRD/CAMARO) FACTORY SERVICE MANUALS: (These are not links - just a list. Click on the...
www.firebirdnation.com
