You may need to highlight to them the foods that contain Gluten and also list the foods you like that you know are Gluten Free. If you are a UK or other EU citizen, you can also inform the restaurant that any food or beverages they buy in will have the correct Gluten ingredients listed on the label, so they can look out for any to make certain.
If a restaurant doesn’t have a Gluten Free dish you like or even doesn’t have any Gluten Free dishes at all, don’t be afraid to ask the chef to make you something off the menu. If you don’t ask you will never know, most reputable restaurants should be happy to accommodate your needs.
Even when a meal is prepared for you, be weary of anything that may have been overlooked, for example most sauces and gravy will contain Gluten, so try avoid having any of these.
Finally if you are uncertain about the competence and customer care offered by the restaurant then simply don’t go there…choose one you are comfortable with trusting.