In a large skillet, brown the sausage over medium heat, stirring occasionally with a wooden spoon, until no longer pink. Break up any big pieces of sausage with the spoon. Don't drain the grease off of the sausage.
Reduce the heat to low. Sprinkle the flour evenly over the sausage and stir until the sausage absorbs the flour. Pour in the milk, stirring constantly.
Cook the gravy, stirring frequently, until it thickens, about 8 to 10 minutes. Add maple syrup and sage, if using. Season with salt and pepper, to taste.
Serve the gravy over warm biscuits.
If the gravy is too thick, you can add 1/2 cup more milk to get it to your desired consistency.