It could definitely be a phase. One of the great things about GSDs and other breeds, is that they can be very independent-functioning dogs...that said, it can be a giant pain in the butt too!
Kneeing a dog in the chest when they are jumping isn't going to get you any positive results, because 1) you are giving them some sort of contact-based attention, and 2) you're teaching them that "hey, if I jump this way, I'll avoid the knee!" I think the best thing you can do is either to simply turn away and ignore, or do what another poster had suggested and reclaim the space. Again, it's important to do this with as little contact as possible, and also without really using any sort of command or words. Your pup will respect your presence and back off...at least after they get the hang of things.
Also, the crate has been a HUGE help for us with our pup. It gives us a break and restores our sanity! Plus I know that it will keep him safe as well. Building a positive relationship with the crate was very difficult at first, but we began with tossing treats in there, and keeping the door open, feeding him in his crate, giving him a kong filled with cream cheese or peanut butter when we had to leave for extended periods of time, and then getting him used to being in the crate even when we were home, by rewarding him every 5-10 seconds he was quiet, and then letting him out. After a while, we would lengthen his duration in his crate, and reward him every 30 seconds, again with very high value treats.
Hopefully all of the information people have provided will help!