Remove excess fat from the chicken pieces. Rub a generous amount of salt, garlic powder, and cayenne pepper on both sides of each piece, making sure each is evenly covered. Let sit at room temperature for 30 minutes.
Meanwhile, in a medium-size bowl combine the onions, bell peppers, and celery; set aside.
Combine the flour, ½ teaspoon salt, ½ teaspoon garlic powder, and ½ teaspoon cayenne pepper in a plastic bag. Add the chicken pieces and shake to evenly coat. Reserve ½ cup of the flour.
In a large heavy skillet heat 1½ inches of oil until very hot (375℉ to 400℉). Fry the chicken until the crust is brown on both sides and the meat is cooked, about 5 to 8 minutes per side; drain on paper towels. Carefully pour the hot oil into a glass measuring cup, leavings as many of the brown bits int he pan as possible. Scrape the pan bottom with a metal whisk to loosen any stuck particles, then return ½ cup of the hot oil to the pan.
Place pan over high heat. Using a long-handled metal whisk, gradually stir in the reserved ½ cup flour. Cook, whisking constantly, until roux is dark red-brown to black, about 3½ to 4 minutes, being careful not to let it scorch or splash on your skin.
Remove from heat and immediately add the reserved vegetable mixture, stirring constantly until the roux stops getting darker. Return pan to low heat and cook until vegetables are soft, about 5 minutes, stirring constantly and scraping the pan bottom well.
Meanwhile, place the stock in a large Dutch oven. Bring to a boil. Add roux mixture by spoonfuls to the boiling stock, stirring until dissolved between each addition. Return to a boil, stirring and scraping pan bottom often.
Reduce heat to simmer and stir in the andouille and minced garlic. Simmer uncovered for 45 minutes, stirring often toward the end of cooking time. At the end, add in the okra if using.
While the gumbo is simmering, bone the chicken and cut the meat into ½-inch dice. When the gumbo is cooked, stir in the chicken and adjust seasoning with salt and pepper. Serve over cooked rice.