Combine ½ cup oil and flour in a large Dutch oven (preferably cast iron); cook over medium heat, stirring constantly, until roux is caramel-colored (15 to 20 minutes). Stir in celery, onion, green pepper, and garlic; cook over medium low heat an additional 45 minutes, stirring occasionally.
Sauté okra in 1 Tablespoon hot oil for 5 minutes. Add to vegetable roux mixture, and stir well over low heat for a few minutes.
Heat chicken broth and water in a large pan. Add heated liquid and next 10 ingredients to vegetable roux mixture, stirring until well blended. Cover and simmer for 2 hours; stir occasionally. (May be prepared in advance up to this point).
While liquid is cooking, prepare the seafood as follows: Peel and devein shrimp; set aside. Pick through crabmeat to remove any shells. If you are going to use oysters, they should be small. Cut large oysters in half.
15 to 20 minutes before serving, heat the gumbo to almost boiling. Add shrimp, chicken, crabmeat and oysters (if desired). Cook for 10 minutes. Remove bay leaf. Add the cooked rice and stir well to mix. Add gumbo filé, if desired. (Note, once filé is added to gumbo, do not boil mixture - the filé will produce a bitter taste.) Yield: 16 cups.