I dont have any Gluten experience myself, although part of diagnosing IBS was a blood test for 'Coeliac Disease'. Which is where the digestive system reacts abnormally to Gluten.
But my partner has spent a few years nannying for Autistic children, where gluten was one of the many absolute no-no's for the kids. I dont think I can post links, but try searching for "gluten" and "elimination diet". Because its a common sensitivity. She doesnt believe its anything to do with the water. Gluten is one of the proteins found in Wheat, Barley, Oats & Rye.
Also once you remove something like wheat or gluten from your diet, youre then far more sensitive to its effects if you do accidently eat it. Youre body loses its tolerance to deal with the offending food.
Lastly something I had noticed, they do sell things in stores like gluten-free muffins/bread etc.. But they dont taste as good, and they are a bit of a boutique item. But they do exist. My partner also used to make casein gluten free items such as Biscuits, Scones & Chicken Nuggets.