Unless I misunderstand slingshotting, you are exploiting - not the gravity of the planet - but the motion of the planet in its orbit. This would mean you cannot slingshot around a planet by moving in a retrograde direction. More importantly (to me) it would mean that you cannot use a slingshot maneuvre to gain speed perpendicular to the plane of the Solar System. It is that last question I'm interested in the answer to. Can you end up on a trajectory outside the plane of the SS by slinghshotting around one of the inner planets?