Preheat the oven to 350°F (175°C).
Place the ground breakfast sausage in a large skillet over medium heat. Cook until the meat is completely cooked and no pink remains, stirring periodically and breaking the sausage into small crumbles as it cooks.
Whisk the beaten eggs with 1/2 cup milk in a medium mixing bowl until the mixture is evenly blended.
Prepare the packet of gravy mix separately according to the directions printed on its package. This generally requires combining the powdered mix with water and heating it to a boil until the gravy reaches a thickened consistency.
Open the refrigerated biscuit package and divide each biscuit into four equal pieces.
Coat the bottom and sides of a 9 × 13-inch baking dish with nonstick cooking spray.
Arrange the quartered biscuit pieces in an even layer across the bottom of the prepared baking dish.
Distribute the cooked breakfast sausage uniformly over the biscuit layer.
Pour the egg-and-milk mixture evenly over the biscuits and sausage.
Cover the casserole with the prepared gravy, spreading it across the top so the layers beneath are evenly coated.
Transfer the casserole to the preheated oven and bake for 40 minutes, or until the eggs are fully set and the surface has developed a golden finish.
Remove the baking dish from the oven and allow the casserole to stand for 10 minutes before slicing. Serve warm.