Why so low compression in my diesel Jetta.?

The fact that you found sealer on the head gasket shows you that this car was worked on by people who had NO IDEA what they were doing. Therefore, you have to assume that nothing was done right. I doubt there is any problem with the rings. 99% of the problems on these engines are the heads and head gaskets. You need to recheck the head for straightness and flatness, and vacuum check the valves to make sure they are actually seating. Clamping pressure is CRITICAL on these engines, The head bolts are not reusable and MUST be replaced EACH time they are torqued down. The same thing is true of the head gasket itself, it must ALWAYS be replaced if it's been torqued down. I bet what happened is they used the old head bolts, which stretched, and they lost clamping force, which let the gasket leak. They may even have used the old gasket! Check that head and put it together right, and it will probably work. The other thing that is absolutely crucial is injection timing. If they did anything to mess up the valve timing, the engine won't run right.