It’s best if your friends and immediate circle of contacts know that you have to beware of gluten. Then with birthday meals, Christmas parties etc. it’s clear that alternatives should be provided for you or else you bring your own food with you. If your friends are in the know, there’s no doubt they’ll be more understanding and ask questions out of interest. This will mean you can avoid being just an outsider. In any case, nothing should come between best friends! And, after all, nowadays it’s not unusual not to be able or not to want to eat certain foods.