Remove excess fat from chicken pieces and rub salt, garlic powder and red pepper on both sides of each piece. Let stand.
Combine onions, celery and green pepper in a bowl and set aside.
Combine flour, 1/2 teaspoon salt, 1/2 teaspoon garlic powder and 1/2 teaspoon cayenne pepper in a paper bag. Add the chicken pieces and shake until chicken is coated. Reserve 1/2 cup of the flour.
Saute chicken in oil until crust is browned and meat is cooked. Drain. Carefully pour hot oil into a glass measuring cup, leaving as many of the browned particles in the pan as possible. Scrape pan to loosen any stuck particles, then add 1/2 cup of the hot oil to the pan.
Place pan over high heat. Using a metal whisk, gradually stir in the reserved 1/2 cup flour. Cook, whisking constantly, until roux is dark brown to black, about 3 1/2 to 4 minutes, being very careful not to let it scorch. Remove from heat and immediately add the reserved vegetable mixture, stirring constantly until the roux stops darkening. Return pan to low heat and cook until vegetables are soft, stirring constantly and scraping pan bottom well.
Place chicken stock in a large Dutch oven and bring to a boil. Add roux 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 a simmer and stir in andouille and minced garlic. Simmer uncovered for about 45 minutes, stirring often.
While gumbo is simmering, bone cooked chicken and cut into small dice. When gumbo is cooked, stir in the chicken and adjust seasoning. Serve over rice.
