Best way to stop gsd barking at other dogs?

Take her back to obedience, the trainer should be able to help you and the practice is good. You could also try this, if she barks, immediately turn around and walk away from the other dog, when she calms down give her a treat and turn back towards the other dog, as soon as she barks, turn around again etc. It will take practice, but they do learn that if they bark and pull, they never get to say hello.