I agree with Wick, the dog mustnt be allowed to play with the rabbit. A GSD is big and heavy. It can accidentally hurt the rabbit without meaning any harm. Dogs can also get excited when they play and see the rabbit as prey, much like when a dog plays with a stuffy toy. They’ll throw the toy into the air and catch it and shake it etc.
This is a bit difficult, because you shouldn’t punish the dog either. If you use any sort of violence it could make the dog resent the rabbit and try to hurt it if it gets the chance. It’s safer to reward the dog for not engaging with the rabbit.
My GSD had had a lot of obedience training before my first rabbit came into my home. I hadn’t planned on getting a rabbit, I found him as a stray. The first thing I did was to make sure my dog understood that the rabbit belonged to the human and was in no way a toy. This was easy bc my dog listened to my commands. This is achievable with a GSD, they are as a breed highly trainable, but it still takes training.