Cook bulk country breakfast sausage in a nonstick skillet, breaking it apart with a wooden spoon as it cooks.
Sprinkle flour over sausage. Stir and cook 1 minute.
Gradually whisk in whole milk.
Add garlic powder, rubbed sage, and salt and pepper to taste. Simmer until thick.
Either refrigerate gravy overnight or line a baking sheet with parchment paper, spread the gravy thin and freeze for 30 minutes.
Preheat oven to 350 degrees. Line a baking sheet with parchment paper.
Pull each biscuit apart into 2 layers.
Place a 1-2 tablespoon spoonful of chilled gravy on a biscuit half and top it with another biscuit piece. Seal around the edges and tuck the edges under somewhat so they are less likely to pull apart.
Repeat for remaining biscuits. Brush top of each biscuit with egg wash.
Bake for 15 to 18 minutes.