Either your gage or your sensor might be bad.
The thermostat should stay closed until the engine gets up to about 180, regardless of load or ambient temp, allowing the engine to come up to operating temp.
Only other possibility would be if the thermostat is stuck open and the engine isn't getting hot enough to register on the gage.
The TM will tell you how to test and what the ohm readings should be on the sensor, which can be tested with a multi meter.
A quick test to check the gage itself, take the wire off the sensor ( the thin sensor with the single wire ) with the main power switch in the prime/run position, momentarily touch the wire to ground and the gage needle should go up. If it does, I'd suspect a faulty sensor
If it does not move, either the gage or the gage wiring has a problem.