My previous rescued GSD who was a senior (in my avatar photo with our 18 year old cat who passed 2 weeks after he did), was very front door aggressive, he would run me over to get to the door. I was so upset & frustrated. After he passed & I adopted my next GSD, he started to get front door agressive. He is also a fearful dog that had a history of abuse & neglect. The front door was one of my most upsetting problems & I needed to fix it because this rescue is much younger & I cannot get through 10 more years of not being able to answer my door. I also have a mix that I adopted who will greet at the front door, but is not at all intimidating.
My trainer taught me to use my knees & body & just plow them out of the way & make the front door my space. She said they plow into us, we can do the same. Not hurting them of course, but nothing wrong with just using your knees to push them out of your way - that is what dogs do to each other.
So now I learned when the doorbell rang, the dogs gave a warning bark. I say "thank you", then "go on" along with using my body, especially knees to push them back. Then I would go to the door, they would move into my space & I would turn around & do it again. This went on many times until they realized that the front door was mine & the guest was mine too & they need to stay back. When I let the guest in & I was ready I allowed them to smell & greet the guests. Then after the greeting I give the guests space & move the dogs away. It is a powerful feeling when you order a pizza once a week & the dogs just give room & I can open the door pay for the pizza & not a peep out of the dogs - they are just in a sit about 6 feet away from the front door.
Do get a trainer though. My dogs were both very dog reactive on leash. I can manage both now on a leash, just today we passed another dog on a leash who was acting up. My dogs started with too much eye contact & I jerked their leashes to distract them. I distracted them before they got to the over excited point & it went very well. I then just turned them around as the dog passed by. I have learned the hard way that if they get too worked up & crazy it is very hard to calm them down. It is a lot of work but it pays off soon enough. GSD's are very smart dogs & want to please us & want us to be in charge.
Good luck & thanks for rescuing a dog in need!