It isn't a chew toy, but we have a toy called The Jolly Egg (the big one). Of course, he tries to chew on it, but he can't and that's the point of it. It's just a really big, smooth toy that is difficult to get a bite on.
My boy's mouth isn't as big as a gsd's, but I think the large 12 inch one should still be big enough to work. The 8 inch was juuuusssttt small enough to fit in his mouth (literally 8.25 would've been too big for his mouth). He isn't dumb, though, and knows if he is gentle it won't fly around. He'll carefully pin it down and try chewing at it, but all that the big one has are some scrapes on it. The smaller one that he can actually pick up isn't in too bad of shape, though. He can't actually do much to the larger end, but the smaller end is worn down.
Note: safen up the inside before giving it. Your dog will be oblivious to everything and run into things as they knock it about. Don't want a vase getting knocked down.