Nothing. This is an insurance issue. They have insurance and are liable if your dog ever gets out and bites someone. So the insurance company has a list of dogs that they won't insure and the GSD tends to be on that list. Usually they won't make exceptions (except for service dogs) because if they make one exception, they have to make all exceptions and then they don't really become exceptions.
Your best bet is to move somewhere that does allow GSDs, but this might be difficult when it comes to apartments depending on your area.