In a large pot or Dutch oven, heat the vegetable oil over medium heat. Gradually whisk in the flour to make a roux.
Stir the roux continuously for about 15-20 minutes, until it becomes a dark brown color, similar to chocolate. Be careful not to burn it.
Add the chopped onion, bell pepper, celery, and garlic to the roux. Cook for about 5 minutes, until the vegetables are soft.
Stir in the diced tomatoes (with juices), seafood stock (or water), Cajun seasoning, thyme, bay leaves, and okra. Bring the mixture to a boil.
Reduce the heat to low and let the gumbo simmer for 20-30 minutes. Stir occasionally to prevent sticking.
Add the shrimp to the pot and cook for an additional 5 minutes, until the shrimp are fully cooked and pink.
Season the gumbo with salt and pepper to taste. Remove the bay leaves before serving.
Serve the gumbo over cooked white rice and garnish with chopped green onions and parsley, if desired.