In a small bowl, combine all the spice blend ingredients: onion powder, garlic powder, paprika, smoked paprika, oregano, thyme, salt, black pepper, and cayenne. Pat the chicken pieces dry and toss them with the spice blend until evenly coated.
Place the seasoned chicken in a single layer at the bottom of a 6-quart or larger slow cooker. Add the sliced andouille sausage on top of the chicken. Scatter the chopped onion, bell pepper, celery, minced garlic, and the white parts of the scallions over the sausage.
In a medium microwave-safe bowl, heat the prepared roux for 15-30 seconds, or until it softens. Gradually pour in about 1 cup of the warm chicken broth, whisking constantly until smooth. Continue to slowly whisk in the remaining broth until the mixture is fully combined and has no lumps.
Pour the roux and broth mixture over the ingredients in the slow cooker. Add the bay leaf and stir gently to combine everything.
Cover and cook on low for 6 to 8 hours or on high for 4 to 5 hours, until the chicken is tender and cooked through.
About 20 minutes before serving, remove the bay leaf. Skim any excess fat from the surface with a spoon. If using, stir in the frozen okra and optional shrimp.
Cover and continue to cook for 10 to 15 minutes, or until the okra is heated through and the shrimp are pink and opaque. The internal temperature of the chicken should be 165°F (74°C) and the shrimp should reach 145°F (63°C).
Taste the gumbo and adjust seasoning if needed. Serve hot over cooked white rice, garnished with the reserved green parts of the scallions.