Unless its a required event, I wouldn't say anything. I used to be part of a crew team that would have spaghetti dinners the night before a regatta. Not everyone participated, so if you really don't want to go, don't.
That being said, it probably
is a fun bonding experience, so I might inquire as to options for those with gluten sensitivity, or (even better!) offer to cook up a few things yourself to bring!
Just don't shove your beliefs down someone else's throat, no matter how scientifically sound those beliefs are.
