This is pretty common however you need to get a grip on it as soon as possible before it becomes a habit and ingrained in your German Shepherd puppies mind and behavior.
I would highly suggest 2 things to start: sign you and your GSD puppy up for a training class and go and 2, buy loads of chew toys to keep your puppy occupied until this teething stage has passed.
Here is an article all about destructive chewing that you should really read too. Good luck
