When you are booking your trip with your Travel Agent let them know you are a Celiac and that you require a gluten free diet. This can then be put on all your booking forms and itineraries that are sent to the various operators that are booking your trip. If you are booking directly with the operators, ensure you let each know separately at the time of booking.
Confirm with Airline Directly
With the increasing awareness and requirement for a Gluten free Meal (GFML) most airlines can cater for the Gluten Free traveler, provided enough notice is given.
That's why it not enough to confirm this with just the Travel Agent. Sometimes the request does not make it through to the airline booking/catering section - It's a bit late to find out at 30,000 feet. It is strongly recommend you contact the airline directly and confirm your request a few days prior to travel. You can then reconfirm on check in if you have any doubts.