To prepare the biscuits: In a large mixing bowl, whisk together 2 cups flour, baking powder, salt, and baking soda.
Add butter to the flour mixture and cut in with the flour using a pastry blender or fork, until the mixture resembles coarse crumbs.
Make a well in the center of the flour-butter mixture and add 3/4 cup buttermilk. Stir with a fork until just combined.
Turn dough out onto a floured surface, and form a rectangle. Fold the rectangle into thirds (like you're folding a letter). Flip over the dough and give it a quarter turn. Then pat it back out and fold it into thirds again. Repeat the flipping and folding process once more.
Roll out dough on a freshly floured surface to about 1/2-inch thick. Then cut out 2 1/2 inch wide biscuits, making about 12 biscuits. You can use a biscuit cutter, but I usually just use a drinking glass.
Cut each biscuit into quarters. Layer half of the biscuit pieces in the bottom of a 9"x13" baking dish. (Set aside the other half of the biscuit pieces for now)
Bake in a 400° oven for about 10 minutes.
While the biscuits are baking, prepare the gravy. Brown sausage in a large skillet until cooked through. Do NOT drain.
Add flour in with the sausage and drippings. Allow to cook for an additional 2 minutes.
Slowly add in milk, stirring constantly to prevent lumps. If you like your gravy very thick, use just the 3 cups milk. For slightly thinner gravy, use the full 4 cups.
Allow the gravy to heat through and thicken over medium heat, stirring frequently. Add salt and pepper to taste.
When the biscuits have cooked for 10 minutes, remove them from the oven. Pour the gravy on top of the biscuits. Then layer the remaining biscuit pieces on top of the gravy.
Lightly brush the top biscuits with the leftover buttermilk (You will probably still have a bit leftover. You don't need to use it all)
Bake in a 400° for an additional 20-25 minutes until biscuits are golden and casserole is heated through.