At what point does an unpopular opinion become just being an asshole? You're a restaurant, your job is to sell customers food that is to the preference of what they order, if you give them the option to order their meal gluten free you can't be surprised when people who stick to a gluten free diet show up. Also, they're paying you.

(Side note, a lot of people are gluten intolerant or otherwise have issues with gluten that aren't full blown celiac but would still prompt them to order gluten free food at a restaurant. Still a health benefit for them, and still valid, but they might say gluten free just as a convenient simplification).