
Heat a large skillet over medium-high heat. Add the sausage and break it up into small pieces with a wooden spoon. Cook for about 5 minutes, stirring frequently to scrape any brown bits from the bottom of the skillet, until the sausage is cooked through.
Reduce the heat to medium-low and sprinkle the flour all over the sausage. Stir to combine and cook for 1 minute, until the flour is fully absorbed.
While stirring constantly, gradually pour in the milk. Cook over medium-high heat, stirring frequently, until the gravy thickens and comes to a simmer. Reduce the heat to medium-low and cook for 5 to 7 minutes, stirring frequently, or until your desired consistency is reached.
Season the gravy with the pepper, red pepper flakes, if using, and salt to taste.
Serve immediately over buttermilk biscuits.