[ecomodded]

The boost gauge reads the turbo's positive psi pressure which moves the needle right.
A vacuum source on it would move the needle left / negative pressure.

Sounds like yours was not marked to go backwards - but it might , you never know until you try , despite being no scale to measure it with you could watch for peak vacuum or what ever. try it on a vacuum source or draw a breath threw it and see if the needle moves backwards.