I'm a coeliac, and I'm horrified by the lack of knowledge or detail given of one or two answers on here. I will spare their blushes, but please, never feed a coeliac or advise others unless you have checked what you are doing. Sadly I've too much experience of being "poisoned" by people who breeze, "Oh, it's easy really, isn't it?".
As a previous poster said, if everything is gf then you'll avoid the real risk of cross contamination and not be worrying.
Some easy options I'd suggest:
Aldi Bratwurst sausages are GF and very good. Buy some ready made mash and a big bag of "stabby veg". (The prepped ones in a cellophane bag, stab the bag and cook in the microwave). If you wanted you could cheer up the mash with a dollop of Maille dijon mustard. Knorr beef stockpots are GF.
Alternatively, M&S Cottage Pie is GF ( ONLY the one I picture below in the brown coloured sleeve). I scatter some grated mature cheddar and thick slices of tomato on top before it goes in the oven.
While you're in Marks'. you could pick up a big tray of their ready to cook roast potatoes with butter to pop in the oven too.
For the vegetarians you'll be on safer ground, but I'd suggest buying some gf pizzas ( many are also suitable for vegetarians) and augmenting them with lots of sliced mushrooms, onions, red peppers, more cheese and Italian herbs. Serve in slices. Put it all on the table and let them dig in!
As ever, check all labels or get BIL to, in case of recipe changes.
It's all a bit processed, but it's one meal, it's tasty and safe and you're on holiday. 😊