If you've got Celiac, then tell them.
If I was the host and a guest didn't tell me about dietary restrictions I'd be pissed. The role of a host is to serve and please the guest, not make the guest sick or leave him wanting. To the OP do what would you expect of your host if the situation was reversed - Miss Manners. Or maybe Emily Post
I agree. If you know in advance you can work around it, and prepare something everyone can enjoy. It isn't that hard to put something together to meet a restriction like coeliac.
If the guest doesn't tell you, but turns up and then doesn't eat the things you've put in front of them, even if it is for good medical reasons, that is apt to have you feeling a bit put out. It would be downright insulting if they did what one poster suggested, and brought their own food to eat instead of what you put on for them.
