My response on dog parks is it depends. On the park, on your dog and you as an owner.
We go frequently to dog parks nowadays because we have a mix who is suitable to be a dog park dog. When I had strong working line schutzhund GSDs we never went to dog parks.
There are some dogs that should not be at the dog park due to their temperament. Many GSDs are not the right dog to be taken to a dog park. There is nothing wrong with a GSD that is not a good dog park dog because there is a lack of fit between the temperament of many good GSDs and the environment. First off GSDs tend to be a dominant breed that is born to control situations. A bunch of playing dogs can be seen by a GSD as akin to a herd of sheep that is unruly and must be controlled. No fun for other dogs and can lead to a fight.
Male GSDs tend to want to measure themselves against other males and figure out rank. Again, a potential dog fight. GSDs even if they do not start a fight will often be the last dog to back down if another dog starts something. Very strong GSDs will not even back down to strange humans which means a potential dog fight can escalate to some people being bit who try to step in to stop it. There are reasons why you never see police dogs and schutzhund dogs at dog parks. Even non-dog aggressive GSDs may not fit a dog park environment for the reasons I listed. And unfortunately there are also some GSDs who are too fearful and shy combined with aggression which means it's a risk to both other dogs and people at parks. Dogs with less than solid nerve and younger high prey dogs can have poor impulse control and not know that a play chase should not end with a bite. Not every GSD is well trained.
GSD puppies before they mature are ok to take to a dog park by 5 months to a year old for socialization if they are solid nerved and are physically well developed and coordinated as long as you are aware of the risk of other dogs and clueless owners, you are very good at reading dog behavior not just your dogs but all the other dogs around, and you are not at all embarrassed to step in before anything serious starts yet not be so paranoid that you misinterpret play as true aggression. True aggression is quite rare in a good dog park.
What is a good dog park? A big park with not too many dogs with a lot of escape routes so a dog never feels cornered. Almost all the dogs and owners have dog park experience, the dogs are solid and not dog aggressive and not too shy, the owners are dog savvy and mature. Sometimes time of day and week can make a big difference. We never go to dog parks between 4 and 7pm because that's when people with 9 to 5 jobs take their dogs to the park, park gets too many dogs - dogs that are hyper from being housebound or in crates or kennels all day. We never go to dog parks on Friday afternoons and evenings because that is when young single people come to take their dogs and hopefully meet another single dog person. Younger people tend to be on their first dog and be less dog savvy. The best dog owners in our parks are usually retirees who come in the middle of the day and have owned many dogs before and are dog savvy.
I have met a number of GSDs at the parks who are good dog park dogs. They are mainly from pet lines or show lines. Low aggression levels and low reactivity. Handler focused and better trained than the typical dog. The thing that immediately sets GSDs apart from other dog breeds is they are always the most handler focused dog at the park and are never really that interested in other dogs. Some GSD owners take their dogs more because it is a large area that is completely fenced where they can hike and throw balls off-leash not necessarily to play with other dogs and their dogs are neutral or indifferent to other dogs off-leash. The only 3 dogs that I have seen in the last few months behave too aggressively at other dogs in our parks have all been GSDs or GSD/husky mixes. Some of the best dog park dogs are pitbulls. I say this as a GSD aficionado.
