Many GSD's are chow hounds and will eat everything in sight.
But many are also VERY picky eaters and will skip meals for days!
I know I've had both, so just learned what was 'normal' for each.
My Elsa was the 'skip meals for days' dogs but I knew if she's willingly snarf down a treat, she's was not sick. Since she literally would lose weight rather than eat and be fine with it, I tended to just accept that as a fact and not hold out and allow her to eat zero food for days.
Instead, I'd just add a small amount of yummy stuff to her kibble with a ton of water. This would thin out and make a gravy of the delicious stuff to coat the kibble, and get her to eat. Opening up the fridge to scan my leftovers would almost always work (almost ANY of my food was gourmet to her
). But also 1/4 of a small can (like tuna fish sized ) of almost any CAT food would tempt her to start up.
Whatever they have to put into cat food to make a picky cat eat, worked for my Elsa. Easy to store and transport these small cans, and store leftovers in the fridge. The huge variety of flavor was important so she wouldn't get bored. And I could buy any of whatever canned cat food was on sale. Since I was only putting such a small amount in mixed with water and dog kibble, it is still the dog food that's giving all the nutrition. The cat food was just to get her to start eating.
Good luck, and if she IS a picky eater, people that haven't got one of them have no idea how frustrating this can be.
Remember, if in any doubt of this being a medical issue, get to the vet to make sure.