Watch the gauge carefully. Note the temp with the engine cold. Then run/drive the car and watch the gauge. Does it move up a bit to where it shows a low temp, like 100, or does it stay where it was cold? If it moves up a bit, chances are the thermostat is stuck open and the gauge is working fine. You can test further by running the car with something taped over the grille to block airflow to the radiator. Like you see truckers do in winter. Does the temp move up? If it does, it confirms an open thermostat.
If the gauge needle never moves from dead cold, or it moves suddenly and/or intermittently, it's an electrical problem in the sensor or gauge wiring.