I am very sorry to hear this. Cancer is a horrible disease whether in people or our pets. 25% of all pets over the age of 10 will die of some form of cancer.
Actually, a gastrointestinal stromal tumor (GIST) is one of the most
commonCommon cold mesenchymal tumors of the gastrointestinal tract (1-3% of all gastrointestinal malignancies) in humans.
I think it is simply underdiagnosed in veterinary medicine.
As you probably know, once a cancer spreads in the abdomen it is not good. So many vessels and lymphatics that spread can be fast and wide spread. The chemo is the only thing that we know will try to slow the progression of the cancer.
Beyond the surgery and the therapeutics you are giving, I do not know of other things that will direct you towards a cure. There may be some holistic treatments that will help, but that is not my area of expertise.
Perhaps another of our doctors will offer other ideas.
I too have a German Shep and I understand the bond we have with these great dogs. If it were me, I would do what is medically proven to help (chemo and supportive) and make Jake as comfortable as possible. Palliative care is now more common in our pets and there is no reason you can't both treat with the chemo and supportive treatments, and make him comfortable. See my blog on this topic.
Good luck to you and keep us posted as to how Jake does.