
Any change of feeding should be done gradually. GSD's especially are known for sensitive digestion, amny doing better on Gluten/wheat free diets. I am sure Nirto has one to suit, but yoiu need to introduce it little by little.
give her 3/4 of her normal feed with a 1/4 ration of the new for a day or two, and graadullay increase the new and reduce the old.
Why are you changing her food, was she in poor conditon on what she had. It is usualy far harder to change a dog used to getting meat to a dry food than the other way around.