I have had this problem a number of times. I'm not gluten-intolerant, I'm a vegetarian, and so many times I've run across gloaters ("oh, this bacon is SOOOO YUMMY. It's so awful you can't have it. Don't you wish you could have some?") or pitiers ("I feel so sorry for you that you can't eat it").
I actually cut back my volunteer hours at an animal rescue because one of the other volunteers, who was highly competitive with me, would bring in a big dish of meat every day and go on and on like this every single time. It was clearly mean-spirited, and the rescue lost a lot of skilled man-hours due to this. I decided I would no longer volunteer past lunch as I'd had my fill of that unpleasant behavior.
Anyway, I have personally not done well at effectively and politely getting people to stop doing this to me. My approach has been to silently seethe usually, which I suppose is polite enough but unpleasant for me personally, and to try to avoid people who do this.
I had a giant falling out with a friend over this same behavior. At every single meal we ate together, she'd bring up my vegetarianism and go on about how awful it was I couldn't eat this, I couldn't eat that... I resolved that the next time she did it, I would, as politely as I could, say, "So you've told me. Let's talk about something else." This caused a giant fight and an accusation from her that I had a rule that you could never discuss anything that you'd ever mentioned before and how she "couldn't live by [my] rules", etc... Our friendship was never the same again (but it wasn't the most healthy friendship, either).