Pooch biting you for giving a belly whomper......whoa.....
First, of the three GSDs I've had, I don't believe I have ever had this type of situation at that particular age ( 9 months )....but if I had...the pup would know something wasn't right in short order. Had a few incidents with pups younger than that which gave me a nip or three which left a mark or two....but I chalked that up to the learning curve of bite inhibition and learning the discipline of a "soft mouth"....but your situation sounds much different.
Just a thought and it probably is of no value and misguided...but for some reason I was led to believe if your pooch accepted the human hand on the soft of their stomach ( vulnerable area )...they essentially "trusted " you...all positive in the "bond". Perhaps, a particular dog laying down, on it's back having the soft of it's belly touched, might be a moment which a dog still not sure of it's "position" could react in a manner which could be viewed as still testing the rank and file.....I'm just guessing from my observations.
There are nuances with many breeds and at times I hear others express exactly what you are stating about their first GSD..." but doesn't like love and affection that all my Rottweilers before him did " GSDs do have a certain air about them...and I am somewhat convinced that a GSD comes to you for all that loving and affection in a slightly different mindset.
The human urge to have their dog come to them for this particular fulfillment, as many breeds do... is many times the norm whereas a GSD and similar mentality dogs show this " love and affection " to those that they deem as their wonderful leader and all those the "leader" displays the same mentality towards or directs the dog to act in proper accordance.
I heard "bonding" in one post..good reply. I also read a few of Chip18's responses and links...all good stuff. I will have to admit I am not as disciplined a person as Chip18 because if my 9 month old GSD nailed me as you describe..I most likely would have reacted rather sternly....and perhaps made a mistake I would have to undo at a later date.
I'd try and win the dog over....in a way that might be different than what you have become used to with your Rottweilers.
This much I do know...when you have the chemistry correct with a GSD, they will offer all the " love and affection " that you seek.....perhaps even more.