I have celiac and just got off the Magic this past weekend. We had YTD and had no issues. Each night our waiter would let me submit my order for both breakfast and dinner the next day.
When I sit down at the table, I would tell the waiter about my celiac. They will pull the order and confirm it with me. They would bring gluten free bread if asked.
I could pretty much order anything off the menu and they would tweak it to make it gluten free. Desert wise I pretty much stuck to the melting cake each night, although one night I did have the Creme Brulle. They did have different gluten free cake options on the lunch buffet. They would cut a fresh piece with a clean knife when I asked. I also had ice cream out of the machine without issue.
This was my first Carnival Cruise after being diagnosed and it was such a good experience, we booked another cruise the day we got off.
Let me know if you have any questions.