To make chicken stroganoff gluten-free you'll want to replace the wheat flour with an all purpose gluten-free flour. I like to use this 1-to-1 flour. For the gravy you'll want to make a slurry with the flour before adding it in. Mix the tablespoon of flour with a tablespoon of water until it dissolves and then add it into the gravy. Doing this will prevent any clumps from forming.