Well if there is no water going into the system............there is 2 outcomes.

1-The gauge is fucked

Or

2-The water is finding its way into the heating system via the hot water side....which i am not 100% sure it can as they are 2 seperate systems.

I would test the gauge first......or even fit a auxillery one on the flow pipe. That way you will see the pressure iin the heating system.

If you dont release the pressure, does it fire the Blow off valve and empty itself???