Not all dogs are appropriate for group play and it is up to owners to be responsible enough to see and accept that. IME this sort of thing is not at all uncommon with mature GSDs, especially males. They are dogs who are very "into" their people and many bond closely to dogs they live with. IME most, at best don't have much interest in interacting with strange adult dogs once they mature. And some behave aggressively to strange dogs, who for whatever reason rub them the wrong way. Of course everyone who goes to dog parks will now say how their GSD loves it
Obviously not all dogs within a breed will behave the same way. I can say in the 9 years I worked at doggy daycare, mature GSDs proved to be one of the more difficult dogs to manage in group play. I love the breed but they were never intended to be happy-go-lucky, never-met-a-stranger sorts.
There is no reason that your dog has to go to a dog park and learning how to bully other dogs is not really great for socialization. For GSDs, "fun time" is being with their people - think about joining a training class for obedience, agility, tracking or an activity that interests you. Take him swimming, biking or do weight pulling or other owner-dog type activities. Take your dog for hikes a Flexi or long lead or take him somewhere to let him run (use a very long line so you can get him back quickly if need be).