You should be able to find enough food in Costa Rica to be gluten free. Corn chips and Tosty brand Bravos corn chips are gluten free for snacks.
Jacks cereal is Gluten Free! Made in CR - http://jacks.co.cr/
Chokos and Roditas (Chocolate/Strawberry) are corn based cereals, the ingredients even say "fibra de trigo (libre de gluten)" = wheat/cereal fiber (gluten free)
Pastas, Cake and Bread Mixes can be found from NNovaRice company in super markets randomly, one had a complete display for them. Made in CR - http://nnovariceglutenfree.com/
Take a travel card in spanish that says you cannot eat gluten to show to waiters at resturants.