Cook the sausage in a skillet over medium heat, turning occasionally, until the pieces are slightly brown and much of the fat has been rendered. Remove the sausage and set aside on a paper towel-lined plate to drain.
Once all the sausage has been cooked, wipe the excess oil from the skillet, being careful not to scrub away those bits of sausage that have stuck to the bottom of the skillet. Add the okra with 2 tablespoons of oil, and cook until it is no longer slimy. Put on paper towel and set aside.
Place the 1/2 cup vegetable oil in a 12-quart stockpot. Heat the oil over medium heat. Once the oil is hot, a tablespoon at a time slowly add the 1/2 cup flour to prepare the roux, stirring constantly. Once all the flour has been added, continue heating and stirring the roux until it becomes a medium brown color, similar to milk chocolate, about 15-20 minutes. Add the onions to the roux, stirring constantly. Once the onions are wilted, add the garlic, parsley, celery, green onions and bell pepper. Strain the seafood stock into the large stockpot. Add the browned sausage and bay leaves and bring everything to a boil over medium-high heat. Then reduce the heat to medium and continue to cook.
Add the okra to the gumbo pot. Continue cooking the gumbo for 45-60 minutes. Add the reserved crabs and shrimp and cook for 15 minutes longer (Option: You can also add 1 pint raw shucked oysters at this time).Remove the gumbo from the heat and stir in the Creole seasoning and filé powder (powder can be reserved for use table side). Let the gumbo rest for 15 to 20 minutes. As it cools, oil may form on the top. Skim the oil with a ladle or large spoon and discard. Fold in the picked crab meat. Taste the gumbo and adjust seasoning with more salt and pepper as needed.
To serve: Ladle the gumbo into shallow bowls and pile some rice in the center. Sprinkle the parsley and green onions over the top. Pass the French bread and hot sauce at the table.