I don't know where the common wire from the condenser is supposed to connect to the air handler. I cannot find a "C" terminal

The GMP is a gas furnace, not just an air handler.
If you can find the control transformer, there should be a small red wire (which eventually connects to the 'R' terminal on the thermostat), a small black, & on the same terminal, a small blue (which is connected to the furnace body). The terminal to which the black & blue are connected is the common side of the transformer. If you make sure you get a good connection, you can simply attach the common from the condenser to the furnace chassis.