Prepare the roux by cooking flour and oil in a skillet over medium heat while stirring constantly until the mixture becomes deep brown and fragrant. This step develops the signature flavor that gives gumbo its rich Southern taste.
Add the onion, celery, green bell pepper, garlic, tomatoes, okra, and seasonings to the slow cooker. Stir everything together thoroughly so the vegetables become evenly coated with the roux and spices.
Place the chicken and sausage into the slow cooker and pour in the chicken broth. Stir carefully so the roux blends smoothly into the liquid and creates a flavorful base.
Cover and cook on low for 6–7 hours or high for 4–5 hours until the chicken becomes tender and the broth develops deep flavor from the sausage and vegetables.
Stir the gumbo before serving and adjust the seasoning if needed. Serve hot over cooked rice with parsley and green onions for garnish.