My concern is that some people seem so afraid to label GSDs (and other protection breeds) as aggressive. It's OK for a dog to have genetic aggression, in fact, with GSD and malinois it is an important component of the breed. It's not a bad word. Aggression with high thresholds, strong nerves, biddability, clear head and training can be a great thing, a terrific thing in fact. Call it fight drive, or prey drive, or defense drive, it is still aggression (my terminology may be different than some).
Certainly, some dogs with aggression are unbalanced, which is why nerves are so important. Either way, balanced or not, aggression needs to be addressed and channeled, but I have never personally seen someone meet success using the treat/click under threshold work with aggression. At least they never get to what I would consider success, which is off-leash control in the real world, within a reasonable time frame, like a few months.
Corrections with a high-drive dog are almost unavoidable unless you can somehow avoid public places, other dogs, and other people your entire life. Bird-dog trainers correct their dogs very harshly for chasing "trash" (deer/rabbits) either with e-collar or check-cord or (old school) chasing the dog down and issuing harsh physical corrections. Same idea- the drive is in the dog to chase and hunt, but they want to channel that drive into controlled hunt of birds only.
Similarly, our breed has drives like prey, defense, fight- which often result in unwanted aggressive behaviors if the handler is unskilled, dog is unbalanced, or both.
A bit off topic, but I see similar issues with people's views on wildlife. We've got a lot more bears, coyotes, and wolves around in the lower 48 than anytime in the past 100 years or so. It is wrong to pretend that these animals are somehow cuddly and would never hurt a person or domestic animal. Of course they would, they are predators! It is only when we recognize a predator for what it is, and respect its ability to do harm, that we can figure out ways to live with these animals as harmoniously as possible. I see a great deal of value in restoring predators to the lower 48 (coyotes aren't leaving anytime soon anyway), but we need to have realistic expectations of the animals and act accordingly.
While dogs are domesticated, and very different from wild animals, it is still important to recognize their capacity for harm and aggression, and not coddle an animal that is showing undesirable behaviors. I like Bart Bellon's methods with e-collar, try to adapt them to my needs. With my non-GSD dog I use bird-dog trainer methods to train recall, because he has very different drives and is a naturally soft dog compared to my shepherds.
But the important thing is to be honest about your dog.