You got here quick! Not what I did! I learned a lot of crap the hard way! I come from Bully world also.
Bull/Mastiff/APBT , Boxer/Pitt, Boxers and have know Rotties my whole life. I always considered them kinda boring looking over sized Boxers myself.
But nice dogs, anyway I found like you that GSD's are very different dogs!
It sounds like you have no bond with this dog? So while a sharp harsh correction is certainly called for! That is not the way I would go!
Build a bond first I would start here:
I just got a rescued dog – what do I do? | stickydogblog
Just start over
Leerburg Dog Training | The Groundwork to Establishing Pack Structure with Adult Dogs
Your guy has "a problem" don't worry about what it is for now! I would do this!
I used to say, I have never done it but working with a rescue Boxer, I did get a chance to try and I was surprised and stunned at the change in behaviour...dog with fear issues.
The two week shutdown
Wheres my sanity: Sit on the Dog, aka: The long down
That's where I would start.