I had a problem with my MF GC 1715 temperature gauge and it proved to be loose screw where mechanical gauge mated with printed circuit board....
Most gauges are basically a volt meter, try disconnecting wire at sender in block and grounding it out to block if gauge moves problem is sender if gauge does not move trace wire back to gauge to be sure of it integrity before you just replace gauge.... Check all electrical connection for their integrity along the way...
Look for item that will look similar to this, it can be on thermostat housing or on head or engine block... Varies according to engine manufacture....