GSDs are a vocal breed, and the standard mandates reasonable aggression. So to expect a GSD to play without growling is similar, in my mind at least, to having a rabbit and expecting it not to hop! As they grow, every GSD I've ever been around has shown some aggression, some totally appropriate, some not so much and needing some guidance. But either way, a GSD that is always "sweet" is not expected, intended, or well bred IMHO!
Rather than leave immediately, teach her what is and is not appropriate...with reasonable limits that is! I mean, what you're saying is similar to, "how do I get my donkey to stop braying"...answer, you don't! Let her be vocal, but guide her as needed. Again, puppies grow up and will go through stages of being more or less "aggressive" (seemingly, because it really isn't aggression!). You have to learn your dog and what their triggers and personal limits are, that's it, in a nutshell. If you need help with that, find a good trainer to help!
It's not the size of the dog in the fight, it's the size of the fight in the dog. Mark Twain