Actually it can be more accurately referred to as a manifold pressure gauge.

0= atmospheric pressure
below 0= vacuum
above 0= positive pressure or boost.

At idle, coasting in gear, or under light load you will see vacuum.
As you accelerate and load the engine you will see it rise from vacuum past the 0 point into positive boost.
There are conversion charts for the metric scale on the factory gauge, I don't know if any have been posted here yet but you can search for them on NASIOC.
The most important reference points for boost are .05Mpa (approx. 7psi) and .1MPa (approx. 14 psi).

 

yup when you're not boosting you'll be in vacuum. its perfectly normal.
what i did to get used to the conversions, is to print out a cheat sheet. slapped that sucker somewhere i could see it, and used it everytime i couldnt figure how many psi's i was at.