Gluten free food is available at least some of America’s airports. Check the options found in these two articles. (Click here for the second one). This article talks about gluten free options at various airports in California. These two articles highlight individual restaurants at the Honolulu and Denver airports.
Be careful, of course. I can think of few things more unpleasant than being “glutened” just before the beginning of a long flight. There is no guarantee that an airport food stand will exercise as much care as the same restaurant “downtown”. My strategy is to eat a meal at home and (if absolutely necessary)fortify myself with pre-packaged nuts or other snack items from one the newstands at the airport.