This answer pertains to the JELL-O brand of fruit-flavored gelatin dessert: Any variety or flavor with the label gluten-free can be considered as safe (according to the 20ppm FDA ruling) for people with celiac disease.
Message from Kraft Foods, the manufacturer of JELL-O:
“Gluten content can almost always be determined from the label, and the most accurate information can be found by checking the ingredient list on our packages. For labeling purposes, Kraft products list all sources of gluten, including gluten-containing grains (wheat, barley, rye, oats, triticale, spelt, kamut, mir, farina) and other ingredients (e.g. “natural flavor (contains rye)”). Kraft has a rigorous protocol to ensure that products bearing a gluten-free claim meet the FDA requirement (less than 20ppm gluten). However, you should know that we do not guarantee that our products are gluten-free because we sometimes purchase flavoring, color or spice ingredients from suppliers who do not list every possible source of gluten beyond what is required by law.”
Homemade jello/gelatins/fruit-flavored gelatins, often prepared for birthday parties or school events, are not likely to contain gluten, but ask as many questions as you can about the ingredients and preparation process before eating. Watch out for potential sources of cross-contact, shared spoons on buffets and gluten-containing crusts sometimes used in gelatin desserts.