Preheat your oven to 350°F. Grease a 9x13-inch baking dish.
In a mixing bowl, combine the biscuit mix and 2/3 cup milk until just combined. Spread the mixture evenly in the bottom of the prepared baking dish.
In a skillet over medium heat, cook the breakfast sausage until browned and fully cooked, breaking it apart as it cooks.
Sprinkle the flour over the sausage and stir to combine. Gradually add 2 cups of milk while stirring continuously to avoid lumps.
Add garlic powder, onion powder, black pepper, and salt. Cook until the gravy thickens, about 5-7 minutes.
Pour the sausage gravy evenly over the biscuit layer in the baking dish.
In a separate bowl, whisk together the eggs and pour them over the gravy.
Sprinkle shredded cheddar cheese on top.
Bake in the preheated oven for 25-30 minutes, or until the casserole is set and the top is golden brown.
Let it cool for 5 minutes before serving.