Celiac technically isn’t an allergy, but yes. Some denominations will give you a gluten free communion wafer. Catholicism, however, requires one that contains wheat although it’s possible to get a very low gluten option that probably won’t trigger a reaction.
Yeah, hopefully it would be clearly labeled. I went to a Presbyterian church one time and at their communion, everyone else got wonderful looking artisan brown bread. They gave me this weird half-and-half cup with brown looking wine and this tiny 1 cm chip of theoretically gluten free cracker sealed on top. It was kinda interesting so I just kept it.
My mom us a retired Episcopalian priest. She said there are gluten-free wafers you can get if someone in the congregation needs them. She also said that it doesn't have to be a wafer, one of her priest buddies once consecrated a pancake to make a point that it doesn't matter what's being consecrated, it's all God's creation.
Note: I am not religious and do not hold these beliefs myself