1. In a large Dutch oven or heavy-bottomed pot, heat the vegetable oil over medium-high heat. Add the chicken pieces in batches and brown them on all sides. Remove the chicken and set aside.
2. In the same pot, add the sliced smoked sausage and cook until browned. Remove and set aside with the chicken.
3. Lower the heat to medium and sprinkle the flour into the pot. Stir constantly to make a roux, cooking until it turns a dark caramel color, about 20-25 minutes. Be careful not to burn the roux.
4. Add the onions, bell peppers, and celery to the pot. Cook, stirring occasionally, until the vegetables are softened, about 8-10 minutes.
5. Stir in the minced garlic, Old Bay seasoning, and Cajun seasoning. Cook for another 2 minutes to toast the spices.
6. Slowly pour in the chicken stock while stirring constantly to prevent lumps from forming. Bring the mixture to a boil.
7. Return the browned chicken and sausage to the pot. Reduce the heat to low, cover, and simmer for at least 1.5 hours, stirring occasionally.
8. After simmering, remove the chicken pieces from the pot and shred the meat, discarding the bones and skin. Return the shredded chicken to the pot.
9. Season the gumbo with salt and pepper to taste. Simmer for another 30 minutes to allow the flavors to meld.
10. Serve the gumbo hot over cooked white rice. Garnish with chopped green onions.