For the roux, in a heavy 10-inch skillet (preferably cast iron) combine the oil and flour at medium-high heat, and cook for three minutes, mixing constantly and thoroughly with a long-handled metal whisk or a wooden spoon until smooth. Reduce heat to medium and continue cooking the roux, whisking or stirring constantly so it doesn’t scorch, until it turns a darkish peanut-butter brown, about nine minutes. Remove from heat and continue whisking constantly until the roux stops getting darker, about three minutes. Set aside.
In a heavy 8-quart saucepan, melt the butter over medium heat. Add the garlic and cook until it just starts to turn golden brown. Stir in onions, sweet peppers, celery and bay leaves and cook and stir for one minute. Add tomatoes, sausage, Creole seasoning, and thyme, stirring well. Cook until all vegetables are tender, about five minutes.
Stir in stock and parsley, mixing well, and bring to a boil over high heat. Reduce the heat and simmer for 15 minutes. Gradually add the reserved roux, stirring constantly until all the roux is thoroughly blended inches Stir in the okra, Worcestershire, hot sauce, kosher salt and pepper. Simmer for 25 minutes, stirring occasionally.
Next, stir in the shrimp and crabmeat and let the shrimp cook about half-way, about two minutes. Add the oysters and bring to a boil, skimming any foam. Continue cooking just until the oysters are plump and their edges curl, one to two minutes more. Remove from heat. Season the gumbo with more kosher salt and pepper if needed.