OP I’m coeliac - Christmas dinner can be totally GF very easily.
Use Knorr stock cubes for gravy with GF plain flour, use a Free From packet stuffing mix or make your own with GF sausage meat (all good quality sausage meat at supermarkets is usually GF anyway) and some GF bread for breadcrumbs - the rest should all be naturally GF anyway.
If you’re having beef buy frozen GF Yorkshire puddings. Buy a jar of horseradish sauce and just check the label.
If it’s lactose intolerance, veganism or severe nut allergies then the lactose shouldn’t be an issue as long as you don’t serve cauliflower cheese or mashed potato, the nut allergy will be fine as if you’re having beef you won’t have stuffing. If somebody is a vegan just buy them a vegan main to heat up in the oven from M&S.
Go to M&S or Waitrose and read the backs of the labels on all their sides, then order whatever you fancy and that suits you all.
If someone is a really fussy eater and they’re grown up ask them to bring their own main course.
For beef I’d just do:
Beef,
Jar of horseradish,
Gravy (easy to do GF),
Carrots,
Sprouts or savoy cabbage,
Roast potatoes or parsnips
None of that should contain allergens, except the horseradish (potentially).