Add the vegetable oil to a large Dutch oven over medium heat.
Stir in the flour and cook (making a roux), stirring constantly (don't stop!), until the mixture thickens to a paste and turns medium brown, about 10 minutes..
Add the celery, onions and bell peppers and cook until softened, 4-5 minutes.
Add the chicken stock, turkey meat, Cajun seasoning and bay leaves.
Bring to a boil then reduce to a simmer. Simmer until the soup thickens slightly, about 30 minutes.
Preheat the oven to 450 degrees F.
Add the file powder, salt, black pepper, cayenne and the oysters (or shrimp) with any reserved liquor. Continue to simmer until the mixture thickens slightly, about 5 minutes.
Divide the rice between 2 6" cast iron skillets that have been greased lightly.
Divide the gumbo between each skillet.
Top with the biscuit rounds, brushing lightly with milk.
Top with 8 cups of gumbo. Arrange the biscuit rounds on top of the gumbo and brush lightly with milk.
Place skillets in the oven on the top shelf. Put a large baking sheet (or two) beneath them on the lower shelf in case some of the gumbo bubbles over.
Bake 202-5 minutes until the biscuits are golden brown, 20-25 minutes.
Let rest 5 minutes before serving.