Has your dog finished teething?
I learned the hard way that GSD are big play biting dogs more than any other dog I've been around. Lots of people on this site were very helpful with my biting issues. I'm sure you'll get some great advice. Some things I tried were the flirt pole & kongs filled with different treats, peanut butter, canned pumpkin,& bananas. My girl still has a big biting issue. We're keeping persistent with the vocal commands to ward her off, even replacing one of my limbs with a toy.
Your dog is still young & all young dogs are very playful. Hang in there I'm sure some terrific advice is coming your way, from someone who is more knowledgeable than I.