its hard to tell from the picture, it looks like the gauge on the right is at zero in that picture.
There are lots of ways to test the low pressure gauge. If you can dump the water from the bubble counter without tipping the tank (maybe use a baster or syringe and then unscrew it), you could close the regulator valve and then open the needle valve all the way with the solenoid on. If that doesn't peg it to zero, then it probably is foobed.
If you read the milwaukee instructions (google), it does say you don't need the right side gauge, just adjust bubble count with the regulator knob and then fine tune with the needle valve.
If you want to run a specific low pressure and the gauge is working, close the needle and open the reg until you get the low pressure desired. then open the needle until you get the bps you want. Milwaukee does not appear to endorse this method, internet rumors I've seen say that the bps is harder to keep adjusted using the Milwaukee recommended method. I don't think I've had mine long enough to tell you but I used a combination of methods and have about 20 operating psi and the bps stays stable for a few days at a time, pretty much.