We had the same issue, but it was the same ped. gastro. doing the undiagnosing. (spell check says that isn't even a word~ because it is beyond stupid!) My daughter was too ill for the endo with biopsy for the Celiac diagnoses. During the gluten challenge for the blood testing she needed to be hospitalized for dehydration for 3 days on an IV. She had more problems about 4 years on the gluten free diet. She was scoped and diagnosed with Eosinophilic Esophagitis. She was put on an elimination diet to see if there were "food triggers" involved. She was then scoped and back extremely healthy. So the elimination diet worked! ~that's when the ped gastro un diagnosed her from Celiac and EE.
If your child isn't eating gluten, the testing will be negative. If she tries the gluten challenge, she could end up being hospitalized for dehydration. (you did say she vomits for 3 days after an exposure)
My advice, find a new doctor that accepts the Celiac diagnoses. (even if the so called doctor is supposed to be the "best" in the area) If any other problems or issues arise then she should get a endoscopy with biopsy.