I would suggest bringing your own prepared food on a microwave safe plate with lid (tupperware was great for that kind of thing).
As for explanation - I would simply say "you don't need to worry about me, I'll bring my own meal. I prefer to do it that way, thanks." I do this for my ds2, "don't worry about ds2, we'll bring what he needs to eat". And no one has ever taken offense to it. I also do not expect anyone to 'get it' or to bend over backwards in an attempt, I'd rather not risk his health.
Or you could eat at the house before your leave and then nibble on a couple of "safe" foods at the table, if you know that there will be something. I can see in certain households salad and veggies could be messed up instantly and they serve them glutened all the time so have no idea not to do it.