I'm new to raising a gsd, so I certainly don't have as much experience as some of the other posters in this forum, but I'll toss my two cents in after watching the videos. My dog is 12 weeks, suuuper submissive and good tempered, but even he still bites and growls, tugging my shirt or pant legs. It's normal what I've noticed is that he mouths more gently usually, but if he needs to potty or gets overtired they get harder. Is she trying to indicate a need to you, needs to play, chew, eat, poo, nap, etc? How much obedience are you doing with her? I'd work on commands like drop, leave it and off so that she knows what your asking. Simply repeating no means nothing to her. I know you said you couldn't walk fast, but maybe get a long line and play fetch? That way your pup can get a bit more exercise? Just make sure you don't toss farther than the line, but my pup runs around like a crazy dog after his squeak toys while I stand still and toss. 10 or 15 minutes of hard play like that, he's ready to potty and nap and much better behaved. I know our pups seem very different and my suggestions may or may not work for you (or you may be doing this stuff already) but at first the growling my pup did freaked me out a bit, is he aggressive? But when I looked at the overall, he's not, it just took me a little longer to figure out the correlation between what his growls, howls, and whimpers all meant.