Are you sure it's an aggressive growl (which IMO the pup is too young to do) or is it a play growl? GSDs are vocal dogs...and I've had to learn that Sabo isn't necessarily barking because he wants to dominate or be aggressive...his barking has "tones" like our voice, and more often it's just "OMG! Come play with me! Do something fun! Look theres a squirrel! OMG! ANOTHER DOG!!!!! MOM!!!!! Lets go play with the dog!!!!!"
I'm starting to read "How to speak dog" as recommended here on the forum, and it's quite informative!