Warm a large skillet over medium heat. Add the sausage and break into chunks with a spatula. Cook until the meat is browned and cooked through. Do not drain fat.
Sprinkle on the flour, stir until dissolved. Cook over medium heat for 3 minutes, stirring occasionally. Be sure to scrap up the browned bits at the bottom.
Slowly stir in the milk. Cook, whisking frequently, until the gravy is very thick and bubbly.
Season with the ground black pepper, sage, and salt to taste. Serve with warm biscuits.
Notes
If your gravy is too thick, add more milk to thin it out. You can do this when reheating the leftovers as well.
Substitutions:
Replace whole milk with half water and half heavy cream. Low fat milk is not recommended as it the gravy will be very thin.
You can use potato starch in place of the flour, use 1 to 2 tablespoons.