The bubbling in the water tank is the over pressure valve releasing the built up pressure, because you obviously have a blockage somewhere in the group assembly, most likely from, as you mentioned you live in an area with fairly hard water and lots of calcium which requires much regular de-scaling or a good water filter.
It does not take much to block up any number of small holes.
If you have mechanical aptitude these machines are really very easy to work on. They are built like old cars, fairly simple to identify parts, lots of space and built to last, so very serviceable. If not you should just take it to a coffee machine service tech.
If that was my machine i would strip it down to bare parts, clean everything and reassemble then reassess the situation from there.
This is a decent pictorial.
Gaggia Classic Disassembly and Cleaning | [protofusion]