For the crab stock:
Combine the crabs, celery, onion, bay leaves, lemons, and peppercorns in a large stockpot with 2 ½ gallons water. Bring to a boil and reduce to a simmer. Cook for 45 minutes, and then strain.
For the gumbo:
Heat 3 tablespoons of oil in a stockpot, then add the onions and celery. Sauté the vegetables for 5 minutes over medium-high heat until they are tender and the onions begin to brown.
Add the crushed tomatoes and okra to the pot and simmer until the moisture has cooked out. Season with the salt, pepper, cayenne, and bay leaves and add the stock to the pot. Bring the soup to a boil over high heat. Lower the heat to a simmer and add the shrimp, oysters, and crab to the stock. Simmer for 10 minutes.
While the gumbo is simmering, make a roux in a cast iron or other heavy frying pan. Place the pan over medium heat, then add the remaining 2 cups of oil and the flour to the pan, whisking until smooth. Continue to cook, whisking constantly, until the roux become a walnut color. If the pan starts to get too hot during the cooking process, remove it from the heat, still whisking, and allow it to cook briefly. Be extremely careful not to burn the roux. Constant whisking will keep the roux from sticking.
Add the brown roux to the soup a little at a time, stirring. When all of the roux has been incorporated, simmer the soup for about 10 minutes until thickened.
To serve, ladle portions of gumbo into large soup plates or gumbo bowls and garnish each portion with a large spoon of white rice.