This is a multi-pronged issue in some ways, so my answer is going to be long:
First, I don't ever believe in being even SLIGHTLY physical (or even negative, using "No's") when a puppy (esp a young puppy) is simply acting on his instincts. And mouthing is completely instinctual.
Now, for redirecting. There's offering a puppy a toy. Then, there's redirecting. Redirecting is much more persuasive. It means saying to the puppy in a language he will understand that the toy is the ONLY option he has and then generously rewarding him for playing with the toy.
When my high drive European lines GSD was a pup, he was a little shark. But my Dh and I always carried stuffed toys in our pocket and had them stashed EVERYWHERE in the house. Pretty much, every time Campeche opened his mouth in that menacing way that only a GSD puppy can, we popped a toy into his mouth and said "Toy! Good Toy!" with a happy sound in our voices. Then we played with him and the toy for a while. Often, we would even give him a tiny snack to go along with playing with the toy.
In other words, we didn't hand him a toy and expect him to play with it on his own. We put a soft fluffy toy in his mouth. I personally like fluffy toys that are bigger than the puppy mouths because the pups can't do anything for a second because their mouths are stuffed full. Soft toys can't hurt new teeth, and they're a little harder to spit out. That gives me a second to get on my knees and rub him all over and thus reward him "Good Toy!" I'm not saying "NO!" Rather, I'm saying "Yes!" to a new behavior.
It's always easier to teach a dog a new behavior that we want than to extinguish a behavior we don't want. Puppies like learning new things, and they like it when we're happy. They love playing with us. If your pup spits out the first toy, quickly put another in his mouth. Then give him another. Then another. Quickly, he'll learn that his only option is to play with a toy.
This showed our pup that there is a better option than attacking or our hands and feet, or whatever other thing he was planning to attack (in my house, it was our senior dogs). Toys mean that we would play with him and that was fun, and he made us happy doing that. What a win-win situation!
It didn't take Campeche very long to figure out that HE could bring US toys. (which we responded to by saying "Toy! Good toy!" and stopping everything to play with him...at least, for a while...) He went from being menacing and causing a ruckus to instigating productive play. How great is that? That's the great thing about GSDs. It doesn't take them long to figure things out. They want to be with us. And they like when we're happy and playing with them. We just have to show them how.
Our current pup started out nearly as mouthy as Campeche, and she's already figured out that it doesn't do much good to attack us. So she brings us a toy, drops into a play bow and if I don't notice her, she follows around and stands in front of me, doing sits and downs, which have also gotten her positive rewards in the past. "Hey! I'm here, and I'm not attacking you! Play with me!" It's pretty cute.
With puppies, bad behavior is always redirected to good behavior in my house. The puppy learned what she SHOULD be doing. How else do they learn what is expected of them? A trained adult knows what "come" means, and they know what they're doing when they ignore me calling. That's when I use the word No, and I go out and bring them back into the house (in the rain and mud, grumbling the whole way). But a puppy? We have to train them what we want first. Nature tells them to nip and bite. We have to explain to them in terms they understand that this is not acceptable. We can do so gently by showing what what we want them to do INSTEAD.
The thing is, pretty much every GSD pup nips and bites. They don't all have dominance issues, so why would someone respond with dominance? They're herding dogs. It's what they do. We just have to train them to do something more constructive with that instinct. You don't need to handle it physically as though it's a dominance issue, just show them they can get your attention and have fun doing it.
Many of the other owners in our all breeds puppy class are covered in scratches and bite marks. We aren't. We aren't geniuses. We just learned from Camper that there's a better way to do it. A GSD pup will nearly always win a battle of wills (or you'll smash their spirit trying). You have to out-think them. They want your attention. Give it to them on YOUR terms.
In addition to redirecting when the pup is mouthing, make sure you're managing you're pup's whole "life situation" so that he doesn't feel the inclination as much. You've probably noticed that there are certain times that your puppy gets more mouthy than others (my puppies always get really mouthy when it's about 9:30 at night. In other words, when they should be in bed. My trainer who specializes in puppies calls this the woo-woo hour, because pups often get the zoomies at this time of night. So, she recommends, put puppy to bed early. No reason to keep "the baby" up just because you're staying up later.
Mine will often get this way about 2pm because they're tired (especially on weekends, when there's a lot going on). So they get 1pm naps. Pups can get too mouthy if they haven't received enough physical exercise or mental stimulation during the course of the day. Make sure he's getting *appropriate* exercise during the day and evening and stimulation. Playing games with you is stimulating. Add in hide and seek, and some "find it" games as well. Physical exercise should include walks appropriate to his age, and what we would call "cardio" exercise -- running, bouncing, bopping around. Unforced, of course, but let pup burn off some of that energy! And lots of undisturbed naps in between for a growing body, and a growing brain as well.
Finally, one final thing, JD's dad raises a very good point. We need to remember to reward our pups (and adult dogs) when they're just hanging out doing what we want them to do (lying there quietly, playing with a toy or chew). We often give them attention (negative attention, but attention nevertheless) every time they do stuff we don't want them to do. But we ignore them when they're doing EXACTLY what we want. When pup is being good, you should whoop it up, get on their level, fork over snacks, possibly a new toy and a minute or two of attention. Then go back to whatever you were doing. THAT is how we train dogs to be good quiet calm dogs.