It sounds like your pup has Megaesophagus. There are several threads on this board about gsds with Mega-E and lots of information on the internet. I am surprised that your vet did not diagnose this? Canine Megaesophagus, Aspiration Pneumonia & Myasthenia Gravis
The first place to start is to educate yourself about the disease and find a vet who is familiar with it. The biggest risk is aspiration pneumonia and so your dog will need to eat a special diet (people either make meatballs or patties for their dogs or some people feed a slurry style diet) and may need to eat in a Bailey chair to minimize the risk of aspiration.
There are several people on this board with Mega-E dogs so you should probably start a new thread in the health section.