Piggybacking on Freaks' comment, have you noticed how people always seem more receptive to food when it falls under a declared theme? If you have a dinner night drawing from a specific culture, suddenly whether or not the dish is vegan and gluten free is unimportant.
So, you could make a bunch of Thai dishes slightly modifying standard recipes to ensure your sauce is gluten free, for example, pad thai, pad see ew, tom yum, fresh rolls, green papaya salad, vegetables doused in satay sauce. Many, many Indian recipes are gluten free without any modification, like dal, chana masala, dosas, aloo saag, gobhi manchurian, vegetable biryani, pulao, baingan bharta. Ethiopian recipes, too, which you should consult kittee. http://kitteekake.blogspot.com/p/papa-t ... -food.html
Also consider what dishes are naturally gluten free or don't require products the in laws would be unaccustomed to the taste of (i.e. gluten free pasta, mock meats).
Polenta- made creamy, maybe with pumpkin or sweet potato, and topped with an herby mushroom chestnut ragout, or smokey, garlicy collards and black beans, or ratatouille; prepared as lasagna; made into mini pizzas.
Moroccan tagine using quinoa in place of couscous.
Faux eggplant parmesan.
Baked vegetables stuffed with risotto or rice/grain/legume pilaf.
Chili, with multiple kinds of beans, with gluten free cornbread.
Stews and soups.
Caribbean inspired coco rice, rice and peas, pastelles, cassava oildown, plantains.
I was assuming they'd be put off by tofu/tempeh, but if they're not, this opens so many more doors!