Both of our gsd's did this when they were puppies. Well, our thirteen week old is still doing it! We stick a toy in her mouth when she's trying to go for us. We also say no or yelp. From what I've heard in the forum, our puppies are known as "land sharks" at this stage, lol! Our two year old gsd grew out of it so I'm sure our baby (and yours) will too.