Is my pressure gauge broken?
I suggest borrowing a pump with a pressure gauge and pumping up your tyres with that. Then try your gauge once you know the tyre pressure. Bike shops and a lot of cyclists have a floor pump because they work better than the little portable pumps than most cyclists carry. And those have a gauge on them that's usually fairly accurate.

You say the pressure release button on the gauge "doesn't seem to make any difference". What do you mean - does the gauge drop back to zero? If not it's definitely busted.