How to replaced water pump and thermostat

This does not sound like a water pump or thermostat issue. This sounds like an engine cooling temperature sensor issue or possibly wiring.

I would remove the sensor and test it or replace it and see if the gauge is restored.

Here is some info about this. Let me know what questions you have. Thanks

https://www.2carpros.com/articles/symptoms-of-a-bad-coolant-temperature-sensor

https://www.2carpros.com/articles/how-a-coolant-temperature-sensor-works