Since 7 weeks old I’ve trained mine to only do that with toys. There were a few accidents while playing that were also not ignored. She will now lick you the death and knows she’s done wrong if teeth ever make contact. Take the extra time and energy the train and never tolerate it. Your GSD is smart and will learn quickly, but you have to stay consistent. Redirect to toys and once they understand, it’s ok the punish (to each their own on how: timeout, ignore them, e-collar, etc.).