Heat a large heavy-bottomed pot or Dutch oven over high heat, add the oil and heat through. Whisk the flour into the hot oil to make the roux. Lower the heat to medium and continue to whisk until the roux becomes a deep brown color, about 15 to 20 minutes.
Add the onions, and stir them into the roux and let them cook, stirring occasionally with a wooden spoon, until the onions are slightly caramelized and the roux is dark brown – the color of chocolate, about an additional 10 minutes.
Add the celery, bell pepper, tomato, garlic and sausage and cook until the celery is soft, about 4 minutes. Add the chili powder, filé powder, cayenne, white pepper and paprika, stirring to combine.
Add the chicken stock and bay leaves and bring the mixture to a boil, stirring occasionally. Reduce the heat to medium-low and simmer, stirring occasionally, for about 45 minutes.
Skim the fat off the surface of the gumbo and discard. Add the chicken and continue to simmer for an additional 45 minutes, until the chicken is falling off of the bone.
Taste the gumbo and season with kosher salt and black pepper. If the stock tastes too much like flour from the roux, add additional water (1/2 cup at a time) and continue to simmer and skim the fat off the top. Remove the chicken bones and bay leaves and discard.
Heat a large sauté pan over medium heat. Add the olive oil and heat through then add the okra and sauté until golden brown, stirring occasionally, about 5 minutes. Add to the okra to the gumbo and stir to combine.
Ladle the gumbo into bowls over a scoop of cooked white rice. Garnish with parsley and green onions.