A DGP IgA most likely points to celiac disease, and since your symptoms are improving after only 40 days on the gluten-free diet, I would guess with confidence that you have celiac disease.
Keep in mind that there is a very very small minority of celiacs that were negative on the genetic tests but still had a positive celiac test. It's unusual, but it can happen. Your diet will not affect the results.
Good luck with the 14 day gluten challenge and biopsy. Try to ensure your doctor takes at least 6 samples for biopsy. I hope you find the answers you are looking for.