Warm a large skillet over medium heat. Add the sausage and break into chunks with a spatula. Cook until the meat is crumbled and browned all the way through. Add the flour and cook until dissolved, about 1 minute. Stir in the milk. Cook, whisking frequently, until the gravy is very thick and bubbly (you can add more milk later if you need to thin). Season generously with salt and lots of freshly ground black pepper. Serve with hot biscuits.