On the CA20s there are two temperature senders on the thermostat cover, where the top radiator hose connects to the engine.
There's a 2-pin one that tells the ECU what temp the engine is, and there's a 1-pin sender that's for the gauge. Clean up the connection on the 1-pin sender and see if that makes a difference.
If it doesn't, ground the wire that goes to it by sticking a bit of sheet metal or a screwdriver in the plug and touching it to bare metal on the car. If the gauge goes up there's a problem with the sender. If not, there's a problem with the wiring in the car or the gauge.