There are such things as celiac informed and educated dieticians. Please don't listen to those who just dismiss every single dietician because personal prejudice. The hospital dietician who counseled me was the most knowledgeable person on celiac I have run across - including my doctor. I was able to ask her any question about celiac and she had answers backed up by scholarly research articals in medical journals. What I wouldn't give 100% credence to is heresay you read on the internet. Do your own research from the experts in your own community.
I supply the sacrament gluten-free bread for the 5 people with celiac in our ward. I would ask the Stake President about this because I believe your Bishop is incorrect in not allowing you to bring gluten free bread or rice cakes for the sacrament.