Season the chicken with kosher salt and black pepper on both sides and set aside.
Heat the olive oil in a large dutch oven over medium high heat. Add the chicken to the pot and cook for 4-5 minutes until browned on all sides. Transfer to a plate and keep warm. Wipe the pan clean with a paper towel and return to the stovetop.
Melt the butter in a large dutch oven or stock pot over medium-high heat.
Whisk in the flour and continue whisking until the mixture starts to turn golden brown and smell nutty, approximately 3-4 minutes. (Do not stop whisking or the roux will burn.)
Stir in the onion, bell pepper, celery and okra, if using. Continue cooking for 2-3 minutes, stirring constantly, until the vegetables begin to soften.
Add the sausage to the pot and stir to combine. Cook for 1 minute longer.
Stir in the garlic, mustard, cayenne, garlic powder, creole seasoning and file powder, if using. Cook for 1 minute longer, stirring constantly and scraping up any brown bit on the bottom of the pan.
Slowly add the chicken stock to the pot stirring to combine. Bring the soup to a boil.
Turn the heat to low. Add the bay leaf, salt and pepper to the soup and stir to combine.
Simmer for 10 minutes or until the soup begins to thicken. Serve the gumbo over rice, if desired. Sprinkle with chopped green onions and parsley for an extra pop of color.