I almost always have some diarrhea issues with a pup when introducing new training treats. A day of gut rest or so and then we go back to gradual introduction. I work until they can tolerate a variety. German Shepherds tend to have easily upset systems. The less the diet varies the more likely the upset with a new item. I just keep introducing slowly until mine can eat almost anything in training without problems. I guess it is doing "gut training" first.
I suppose there are certainly individual dogs that can never tolerate this or that, but all of mine over the years have become as tolerant as Labs of new food and treats because I don't restrict their exposure to different treats..... salmon, liver, NB rolls, hot dogs, various cheeses, Biljac frozen, steak, chicken, liverwurst, tortellini, etc are on the training menu after slow intro here. You have introduce use just a little bit at first.