Peel shrimp and save heads. In large pot combine heads, two quarts water, one tbsp salt and liquid crab boil. Boil for 10 minutes. Drain and reserve liquid.
In a large pot heat oil to medium heat and fry shrimp until pink, stirring occasionally. Add onion,garlic and file, thyme and bay leaves and fry for 10 minutes.
In a large frying pan, fry okra for 5-10 minutes stirring constantly. Add okra, tomato sauce and tomatoes to pot of shrimp and seasonings. Add reserved water. Make roux and slowly add to gumbo.Add crab halves and simmer for 35-40 minutes uncovered.
1/2 cup oil
1/4 cup flour
Heat oil to medium heat in small frying pan. Add flour, stirring constantly. Fry until flour is dark brown. Add a teaspoon of oil as needed to keep flour moist.
Serve over rice with hot French bread