Don't tell him to COME unless you can enforce it and he will come every time. Otherwise it teaches him that he can choose to listen to that. I use the name until we have COME down on lead, and then on lead with distractions, and then off lead, and then off lead with distraction.
As for coming to strangers when they call his name. Little puppies will. But as the GSD gets older, they are less likely to go all the way to a stranger and listen to their commands.
I wouldn't worry over much about it. If the dog is never alone, your GUNTHER! should bring him to you even if they boy on the other side of the fence is yelling cookie or steak!
Bear Cub, Hepsi-Pepsi
Cujo2, Karma Chameleon
Ramona the Pest, Kojak -- who loves you baby?
Tiny Tinnie, Susie's Uzzi, Kaiah -- The Baby Monster.