Preheat oven to 350°F. In a large oven-proof dutch oven, combine flour and oil. Bake for 2 to 4 hours, stirring frequently, until roux turns milk chocolate brown. Finish gumbo on stovetop, adjusting time based on oven, cooking vessel, and elevation.
Sauté celery, onion, bell pepper in dutch oven with roux for 8-10 minutes, stirring frequently. Add garlic and creole seasoning for 1 minute until fragrant.
Add chicken stock, bay leaves, thyme, salt, pepper, and creole seasoning, cook uncovered for 45 minutes. Sauté sausage in a large skillet with 1 tablespoon oil.
Simmer gumbo, sausage, chicken, okra, and broth until thickened, then add bay leaves, remove excess grease, and serve.