I used to order the gluten free meal (in Y long-haul), and found that the meal was also used for dairy free, diabetic, low calorie, and possibly other options. It was usually a basic fish or chicken with rice, and gluten and dairy free (and flavour free as well). The starter was some sort of salad and dessert was usually a few slices of fruit.

If you are strictly avoiding certain things, I recommend bringing ample snacks or additional food as I've had them serve me things that were clearly not gluten free (package marked wheat).