I'm coeliac, & have & still willing to cruise P&O, Princess, Royal Caribbean & NCL.
As Geoff/arxcards said you need to advise them of your dietary needs. P&O & Royal had plenty on display in the buffet area stating gluten free. If in doubt about anything, you ask questions. The cruise lines want you to be happy & are more then willing to work with you.
The only cruise line I was sadly disappointed on was Blue lagoon cruises in Fiji in 2022. They were atrocious regarding food intolerances, regardless of plenty of notification.