That guage is the factory boost guage -7 is negative (about) 1 bar (14psi) and the last time it was on zero is becuase you didnt have it connected or blocked off... if its still not connected and showing -7 you have a massive leak...
Your problem could be one of many and you havent given much detail on set up, i.e. spark plug type, coil packs, afm, fuel pump etc etc
Also other symptoms like missing or pinging