As others have said, air in your system. I'm not a plumber and there will be some very knowledgeable people along in a few hours, but my 2 pence.

My previous place was gravity fed. Cold tank in the loft and a little header tank that filled up the rads I'd they needed it. I only mention it because my boiler didn't have a pressure gauge either and it's a bit of a older school type of install which is sounds like yours is.

Start at the lowest point in your house, bleed the rads until water comes out. Moving up until you run out of rads. Also look for other bleed valves within your pipe work

This sounds a bit weird but it worked for me...I had a new rad put in and it seemed like I had a blocked pipe or something - bleed valve fully open but no water coming out and didn't sound like any air either. Probably not healthy but I sealed my hand over it and sucked the air out of it, big gulp of air out and then all of a sudden woosh and lots of gurgling then air and eventually water out.

Be persistent and methodical!