Very useful. Each wheel has its own shut off valve. This could be on the hub outer or near to the tyre. Commonly 6mm square peg control. To operate turn like a tap, clockwise for off, anti clockwise for on.
To inflate the tyres. Go around each tap and turn on (anti clockwise). Go back to the cab, the gauge should show the combined air pressure of all tyres where the valves are opened. If some tyres have more air pressure than others, by undoing the valves, pressure will all equalize.
Now operate the air lever. Position varies depending on model, usually this is near the gauge, but with the Gaz its its close to the gear shift. If you need to inflate, move lever either to left or right, this varies again depending on the model and how it was piped up. Don't panic if the needle goes off the gauge, this just indicates there might be a restriction in the air line, or you haven't switched the tyre air taps fully on. Commonly you can also still find normal car valves fitted. I tend to remove them, air pressure will not pass by these valves if installed, to explain this.
When you inflate your car tyre at a garage, you push the tyre inflater end on to the valve, it pushes the valve in manually. Air pressure has nothing to do with controlling the valve. So as mentioned check these valves are removed.
To deflate the same applies, operate the lever to dump air. Its useful once you have altered air pressure to turn the taps off. unless you plan on vary pressure for off road purposes. Else you might find flat tyres in the morning, but this is no problem. Just inflate again.