Next time they ask, just ask, "Why, how do you, a non celiac patient, think you know so much about my disease?" Have them name the major 4 or 5 additives you have to avoid but they have NO CLUE what they are (modified food starch, yeast extract/malt extract, caramel coloring, MSG) and they'll quickly be quiet.
Fortunately, in the US, modified food starch, yeast extract, caramel color & MSG do not contain gluten. On the rare occasion MFS is made from wheat, it will say something like MFS ( wheat) or modified wheat food starch. Of course, malt contains gluten.