Find restaurants that are good for GF (try putting things like "gluten free restaurants in Barcelona" into google. Or "tripadvisor gluten free Barcelona").
Try to contact the restaurants that you are interested in directly (facebook can be great for this or email or the restaurant's own website if they have one) to check exactly what they can do and make reservations.
Print off gluten free restaurant cards in the appropriate language (these tell the chef what you can and can't have). Also, always go prepared with snacks in case of problems - particular at airports and on plane.
You can take packed lunches through security and onto airplanes as long as they aren't going to be classed as liquids (e.g. no soups or yogurts). Avoid nuts in case of fellow passengers with a nut allergy (when eating nut products will be banned). We find that chocolate spread or jam sandwiches travel extremely well (no refrigeration necessary).Or cheese and pickle if you can keep them cold long enough eg freeze sandwiches until leaving the house and use an ice pack and small cool bag but put the ice pack into suitcase just before the suitcase is checked in. (peanut butter would be good on car journeys if there are no nut allergy sufferers in your car).
We tend to take packed lunches everywhere unless we are convinced there will be GF food available (even then we take snacks). Sometimes we find that we don't need the sandwiches because an amazing GF opportunity appears - then they get saved for later or the local ducks or birds get fed!
Remember - Holidays are about so much more than food!