What you want required no more events from my capx.
From my capx, how about you stretch the "down arrow" to fill the upper half of your screen, and stretch the "up arrow" to fill the lower half?
Now, place the solid in the middle, and voila, you will have what you want in no time.
This way, we won't be dealing with line of gravity, and you can specify directly which area should be up or down by placing and stretching these gravity arrows. Additionally, you can turn these arrows invisible for aesthetics purpose.