no they have a sensor and a switch. The sensor is IN the head, the switch in the thermostat housing. The switch turns on the horn. There's a horn under the rear fender that goes BEEEEEP when the engine is overheated.
Very likely to be experiencing a typical G1800/G1900 problem--poor wiring connection and/or corrosion. Under the dash are several connectors. Pull them apart and CLEAN them. Then there's another one WAY underneath the tractor's frame, like under the radiator. That one is a bear to get at, easiest to pull the deck off and lay underneath. Also check your GROUND, as they're also notoriously ill. There is a ground cable (kubota calls it "earth cord") from the battery to the frame. THEN there's a short jumper between the engine and the frame, remove both, clean the connections, clean the frame, the bolt head, threads, and then reinstall all of em. If that stuff don't solve it, pull the cluster out and check the cluster connector for dirty/poor connection. If all of that is verified "good", now you can remove the sensor (not the switch, unless you just want to hear the horn), ground the wire connector to the engine somewhere and see what the gauge does. If gauge does not move, gauge is likely at fault. It should go to full hot with the wire grounded. If it does, the sensor may be at fault and it is easy to test using a pot of distilled water, a thermometer in the water, and a way to heat the water. You will measure the sensor's resistance as you heat the water. Resistance should slowly and steadily decrease as the temp increases.