OP I totally get this, especially how awkward it is to refuse something when someone has gone to the trouble of making it 'gluten free'. Obviously in an ideal world your DP would just not eat there like people are saying but equally it's not unreasonable for someone to want to join in family events and not have to bring their own food so I do think there may be some middle ground options before fully accepting he cannot eat their food if they are genuinely nice people willing to try.
Things I'd suggest:
Sit them down and have a good open chat about the long term effects of glutening and make sure they understand they are doing long term damage. In a nice, not guilty tripping way, it's just factual. It's him and them working to solve a problem, not him Vs them.
Check the ingredients they've used. I do this but say I'm sure it's fine but do you mind, it makes me feel better. Real friends don't mind and actually appreciate it. Ideally check with them before they start cooking. One of my friends likes to send me a photo of the ingredients before she starts cooking which I love.
Give them ideas of easy gf meals with the associated caveats e.g. risotto (check stock cubes), a roast (check gravy, no Yorkshires or better still gf yorkshires, no butter if the butter has crumbs in), roulade (just double check ingredients), most curries (just double check), rice noodle stir frys etc.
Offer to cook alongside them a few times to offer tips for not cross contaminating or ask if they'd like to cook with you.
If you are having, for example, pasta, ask if they are willing to all have gf. It tastes pretty much the same when hot (gross when cold) and in lots of places costs the same. No one ever notices if I cook gluten free pasta though not all brands are as good as each other. Sainsbury's own brand is great. But if they do one pan of each its a faff and the cross contamination risk is so much higher.
That may all seem like a lot to ask but if they genuinely care and want to learn it's really not. A lot of people on here saying it's a big ask to completely avoid cross contamination but I have friends that cook for me in their gluten-containing houses and my house actually has gluten in (I'm the only coeliac) and it's not that hard, it just needs open conversation and a willingness to learn. If anyone genuinely wants to best support their coeliac friends and family I'd suggest cooking naturally gluten free things (curries, meat, potatoes, rice, fruit etc), check ingredients for barley, wheat or rye (someone mentioned not knowing malt vinegar had gluten in but if they checked the ingredients it would have said barley) and if in doubt just ask the coeliac for their opinion.