On my Ferroli 77 FF boiler the pressure gauge indicates the water pressure within the central heating system (radiators, pipes, etc.), not the gas pressure as I first thought. At a recent service I was told that it should be kept at around 1 bar (atmospheric pressure) and your heating may not be working because the pressure is too low (this probably won't affect the hot water side of things). On my system there is a tap, fed from the mains water pipe, which I can open to add water to the central heating system, and then close again when the desired pressure is reached. If you add too much and the pressure is too high you can just bleed a radiator of water until the pressure drops again. I hope this helps.
Rick, January 2008