Leave the thermostat in the mv/c position to run the unit on its internal batteries.  The C wire cannot be used to power the thermostat on 12VDC systems.

Mark your wires using the letters enclosed before you take them loose from the old thermostat and then connect as shown above.

If you have lost track of your wires, try to identify the heating cable of 2 wires, separate from your air condx cable of 3 or 4 wires.

To test for heating wires, connect the two wires together and the 12VDC heater fan will come on within about 30 seconds. The gas will then turn on within 30 seconds after the fan comes on.  Once you have found the heating wires, the remaining 3 or 4 wires are for the air conditioner. 

The GemTech thermostats do not have a fan HIGH/LOW switch so chose which you wish.  Connect just the G wire to G for low, or connect both the G wire and the H wire for high fan.