There would be four areas that could cause this.
1. The sending unit on the engine.
2. The wiring between the sending unit to the gauge.
3. The power driving the gauge.
4. The gauge.
You can start with the sending unit on the engine. Unplug the violet wire on the sending unit. Using an Ohm meter (hopefully analog) should be reading around -75 Ohms to a clean ground. With the engine warmed up should show a -10 Ohms reading to ground.
Test the impedance reading of the temperature unplugged violet wire from the sending unit connector on the gauge unplugged. That Ohm reading should be 0 Ohm impedance. If you are not seeing any readling, you may want to check the wiring passing through the bulkhead connector.
Make sure the black power feed from your voltage regulator is plugged into the temperature gauge. It may simply be unplugged. If you want to confirm power to the gauge, set your meter to read DC and watch for a one second pulsed 12 volt reading with a stock voltage limiter or a steady 5 volts with a repop voltage regulator.
The gauge unplugged should read around -18 Ohms. If there is no impedance reading at all, it would be your gauge has failed.
Jim
