You can use supplements, feed the best of foods, and groom regularly, but some GSD's just shed more than others and people that haven't owned multiple GSD's at the same time usually don't understand how true that is. So you'll get a lot of recommendations for "I use this and my GSD doesn't shed that much..." and it either means: A) their dog just simply doesn't shed as much as yours or B) their standards are lower.
We have three german shepherds. I won't count the malinois since he hardly sheds period....Out of the three GSD's I have, one truely is a twice a year blow coat kind of girl. She doesn't shed that much outside of the twice annually. One sheds horribly YEAR round with a really bad shed twice a year in addition. The other sheds somewhere between the other two and I swear has probably 4x's a yr that she sheds badly.
My mom has two GSD's. One doesn't shed hardly at all, I mean hardly. He has a vry short tight coat with a thin undercoat. The other has the thickest undercoat I've ever seen on a dog. It doesn't seem like it when you look at her, but she sheds buckets. When she gets a good bath and blowout, it looks like she's lost 10 lbs because all the loose fluffy undercoat is gone.
Worth noting, my dogs are fed a good quality diet, her dogs despite my talking to her regularly eat (usually) pedigree. The dog of hers that sheds a ton, we had when I lived with her years ago before buying my own house, getting married (yada yada
) and that dog ate a grain free diet back then. She still sheds exactly the same now on pedigree as she did on a grain free diet when I lived there and was buying the food. The other dog of hers that doesn't shed at all hardly was a foster dog I had after I had already moved out. she adopted him from her after his HW treatment was finished as a companion for her female, and his diet went from great (not grain free) to pedigree. He still sheds the same.
you also have to watch here when people post about how much their dog sheds, because a lot of times newer members with young dogs (under a yr) post about how their GSD doesn't shed much. IME, GSD's don't really start shedding until their adult coat comes in after 6 months old, and really don't start shedding until around a yr and up.