If I were in this situation, I would ask the person(s) involved what their preference for a main course was -- probably end up with two different dishes but something the entire assemblage could enjoy. My initial thought was deep fried turkey and non-creamy salad/cole slaw.
A hearty soup or beef/vegetable stew comes to mind. I have done this or a beef/bean chili for DS (who is also GI) A Gluten free flour mix can be used for thickening the stew if desired. Easy to make ahead too, and just reheat.
There are easy Gluten free mixes for corn bread, as a side. I don't think the mix has milk in it, but check? These used to be only available on line or in health food stores, now most large grocery chains carry them too.