Place flour in a 15- x 10-inch jellyroll pan. Bake at 350° for 1 hour and 15 minutes or until a dark golden brown, stirring every 15 minutes. Set aside; cool.
Peel shrimp, reserving shells; devein, if desired. Set meat aside.
Combine shrimp shells and 5 1/2 cups chicken broth in a large Dutch oven; bring to a boil. Reduce heat, and simmer 15 minutes. Pour mixture through a wire-mesh strainer into a bowl, discarding shells. Keep broth warm.
Coat a large Dutch oven with cooking spray; add onion and next 3 ingredients, and sauté until vegetables are crisp-tender. Add sausage, and sauté 5 minutes. Add tomato paste and 5 1/2 cups warm broth, and stir well.
Spoon browned flour into a medium bowl. Gradually whisk in 1 1/2 cups chicken broth. Add to broth mixture, and simmer 1 hour, skimming as necessary. Add remaining 1 3/4 cups chicken broth, salt, and next 6 ingredients; simmer 5 minutes or until okra is tender. Discard bay leaves.
Drain crabmeat, removing any bits of shell. Add crabmeat, shrimp, crawfish, and oysters to gumbo; return to a boil. Reduce heat, and simmer, uncovered, 10 minutes or until shrimp turn pink and edges of oysters begin to curl. Serve over hot rice.