Hmm I'm on the fence here as I find this a very difficult subject to broach if i stay with people and more often than not end up in pain/bloated/feeling sick because I don't want to be seen as the kind of faddy person you seem to think anyone without celiacs but not wanting to eat gluten must be.
I am not coeliac or have crohns or anything. What I do have is terrible IBS caused by FODMAPS (sugars) in food that I can't digest properly. This causes them to ferment in my tummy and I bloat up like I'm 10 months pregnant, feel sick and then, erm, the bloat has to escape...(accompanied by a runny tummy, nice).
I hate having to go through this explanation when trying to explain why certain foods are tricky for me. Unfortunately this problem is caused by a wide range of foods including wheat, onions, garlic, milk, yoghurt (cheese is higher fat so is generally OK), most legumes (luckily not lentils), apples, pears, citrus fruits, too many vegetables in one go...
In short, it's a nightmare and sometimes I will have a small amount of problem causing food as I feel like such a pain giving this awful list of foods to avoid, that then I probably make people feel like I'm picking and choosing when I am/am not able to eat things.
In reality, I'm just suffering the consequences later in silence.
So if anyone said to me they didn't want to eat a certain food, I'd try to accommodate as much as possible.