If you switch the ignition on without starting the engine it should show zero on the gauge.
If you start it up and let it idle it will show a negative pressure as the engine is sucking air in from the inlet manifold.
If you rev it up the turbo's will start to push air into the inlet manifold , the needle on the gauge will move from a negative pressure, past zero and will show a positive pressure, the final positive pressure is obviously dependent on how far you put your right foot down.
A stock car under full boost will give approx 0.8bar or 23 inHg
A BPU modified car under full boost, most of us limit it to 1.2bar or 35inHg.