Brown the sausage in a Dutch oven in oil over medium heat. Remove the sausage to paper towels, leaving the drippings in the Dutch oven.
Sprinkle the chicken with 1/2 tsp salt and 1/4 tsp pepper. Brown the chicken in the drippings left in the Dutch oven; transfer the chicken to paper towels, reserving the drippings in the Dutch oven.
Add the flour to the drippings in the Dutch oven; cook over medium heat, stirring constantly (do not leave the Dutch oven -- you must stir constantly and watch the color) until the roux is caramel-colored (about 5 minutes).
Add the onion, green pepper, celery, and garlic; saute 10 minutes or until the vegetables are tender.
Add the reserved sausage, chicken, water, chicken stock, and salt and pepper to taste. Bring to a boil; cover, reduce heat, and simmer 1 1/2 hours or until the chicken is tender.
Remove the chicken; set aside, and let it cool enough to handle.
Add the oysters (optional), okra, green onions, and parsley to the Dutch oven; cover and simmer 10 minutes or until the edges of the oysters begin to curl.
Bone the chicken, and coarsely chop the meat; add to the gumbo. Remove from the heat and let it stand for 10 minutes. Serve the gumbo over hot basmati rice (popcorn rice) in individual serving bowls. Sprinkle each serving with gumbo file.