Our GSD pulls when out of her "zone", also. We use the Gentle Leader and training to solve this problem.
The Gentle Leader must fit properly. The loop end rests on the muzzle right below the eyes and the buckle part should fit tightly behind the head. It will sit high on the back of the head. Jax will not like it; at least Kyla doesn't. It doesn't hurt, though, it's just controlling. It works to guide the head and it will diminish pulling, but without proper training, some pulling will still occur.
The best training for pulling takes time, patience, and repetition. First, train Jax to "heel" (if using German commands, it is "fuss", pronounced more like "foos"). Use the Gentle Leader to guide him into position at your left side. It is important that it is the dog that is moving and not you. Position your dog, not yourself. He should sit at your left side whenever you are not moving, so at heal, he should come to your left side and sit. If he responds to the command in the right spot, wonderful. If not, when he reaches the position of heal, gently raise the leash. This will guide the head up, which encourages the dog to sit.
When you begin to move forward, the dog should be at your side, not leading. Some dogs will respond well to a simple pull of the leash when wearing the Gentle Leader, but, in my experience, GSDs need something a little different. There should be a bit of slack between your hand and the dog. If you are holding it tight and there is constant tension on the leash between you and the dog, you are doing it wrong. As long as there is slack and the dog is walking properly at your side, keep walking. When the dog starts to move ahead and the leash becomes at all tight, stop. If the dog is not at your side where he should be, give the heal command and guide him into position (again, remember to position the dog, not yourself). He should be in the proper spot at your left side and sitting before you will go again. Repeat this every time he moves ahead or pulls. It is very important to do this in areas that are unfamiliar or areas where he is known to pull. GSDs tend to associate behaviors with a location (this is how I have to behave in the house but not outside, for example).
It will take time, but if you are persistent, consistent, and patient, it will work. After a few days, you will probably notice the dog paying more attention to you, looking to you more often. This means he's getting the idea and watching for your cues to know where he should be. Also, don't make the mistake of thinking that once he is doing it right he will always do it right. He will probably need regular practice to make sure he stays fresh on the commands.
