sounds like your water pump ain't pumping, don't drive it till it is!. could be as simple as a new fan bewlt or might even need a pump
If the coolant level is okay, I would suspect a restricted heater core. Try flushing the heater core.
Your low on coolent! system set up to protect engine first so routs coolent away from heater.
Temperature gauge may drop when heater turned on because additional coolant from heater core is introduced to cooling system - usually this coolant is cooler than what is currently circulating
This would indicate that the system is low on coolant when the gauge is hot. Not enough coolant to service the heater core.
Because it is a Pontiac. Sorry, man.
temperature gauge not working can be caused by a bad temp sensor in engine, a bad gauge in dash, or a bad fuse
thermostat
change t.stat
If the temperature gauge is reading below normal operating temp after driving a little while it may be that your thermostat is stuck open. I replaced my thermostat and it fixed mine.
Check coolant level
A short between the heater switch and temperature gauge.
thermostat maybe stuk open fit a new one