Sunshine Seasons in The Land is generally considered one of the places with some of the best CS in any of the parks for folks with food allergies. They have so many options in general that it's easier for the chefs to make or modify food to make it allergy and/or intolerance-safe. (Sunshine Seasons is Disney owned & operated.)
It's World Showcase where you have the most to worry about. The majority of food locations there are not Disney owned & operated. That said, I know we have plenty of folks who are GF (or have other allergies/intolerances) and have had luck with locations around World Showcase. This includes some of the locations that are not Disney owned & operated. I can think of positive reviews from Mexico, the Rose & Crown, and Le Cellier.
You might want to look for reviews from clanmcculloch. She and her family have a much longer list of allergies/intolerances than just GF, but she's written wonderfully detailed reviews of places they've eaten, including all over EPCOT.