To test the gauge, disconnect the yellow/green wire from the coolant temp sensor (it's a one-wire plug directly under the distributor), and then hold that wire to ground with the assistance of a friend or a jumper wire. With the ignition ON and that wire grounded, the temp gauge should move toward full Hot.

If it doesn't, check for a break in the wire and make sure the plugs are fully seated in the back of the cluster. If all the wiring looks good, replace the temp gauge. This can be done without replacing the whole cluster, but more than likely it's cheaper to just replace the whole thing.

If the temp gauge checks out, replace the coolant temp sensor.