Heat large skillet over medium-high heat. Add sausage. Crumble and cook until well-browned. Season with salt, pepper, sage and garlic powder.
Remove cooked sausage from pan and set aside. Melt butter in same pan. Add onions and cook until tender, about 5-6 minutes.
In a small mixing bowl, combine flour with ½ cup of the milk. Return the cooked sausage to the pan and add flour mixture and remaining milk. Stir to combine. Season to taste.
Cook until gravy thickens and is bubbly, about 6-8 minutes.