I use it to check the air pressure in soccer balls before a match. Most of those recommend a pressure of approx .75bar, or approx 10-11psi.
Sounds like he is at 100 bar.
Since he is getting a release of air when he bleeds it means the tank is filling the hose, but not opening the foster to fill the gun.
edit - My guess is that the gauge has "1", "2" and "3" on it but then at the bottom is really small letters it has "x100". Just like the Benjamin gauges (except psi and x1000)