Another thing to consider is that Gizmo is nearing full physical AND mental maturity. Even without the negative experience of being attacked, a mature dog typically begins to "assert" himself more frequently with other dogs in social situations.
My Jibini was 100% playful at the dog park from puppyhood until just shy of 2 yrs old….we visited the park 3-4 times a week and until he hit 2 yrs he never engaged in assertive behavior or fighting even when another dog tried to start a fight.
Gradually after age 2 he became more & more assertive, several times I had no choice but to take him home. After a while we became very selective about WHEN we went to the park and WHO we went with. I was fortunate to live in Tampa at the time, where Jibini could enjoy playing with a group of familiar Basenjis who met every Sunday at a particular park. The Basenjis always stuck together in an almost exclusive group, ignoring most of the other dogs & staying in one area of the park away from the main crowd. If we attempted to go to the park without the Basenjis present, Jibini would inevitably start a fight with a bigger dog- without fail.
It was during this time that a couple of knowledgeable folks pointed out the fact that Jibini had reached maturity, and that could very well account for his newly developed "attitude". Since then I've had a lot more dog-related experience & have seen "maturity" trigger assertive behavior between 1-3 years of age in a lot of other dogs.
Just a thought. Keep working with him if you're seeing an improvement, of course...you probably CAN make an improvement with his obedience & possibly modify his fear reaction. Just keep in mind that this behavior probably won't entirely DISAPPEAR, and may not be isolated to just German Shepherds & similar dogs. You may have to be vigilant with him forever, he may react when you're not expecting it, you may find yourself leaving the dog park more often to avoid trouble, etc.
My male is 10 years old and I still can't always predict which dogs will trigger aggression & which dogs he'll want to play with. Rarely he'll have such a severely aggressive reaction to another dog that nothing I do can re-direct or distract him, and the only option is to remove Jibini from the situation altogether.