This would actually be very cool. A gravity behavior that has the same controls as the gravity in the platform behavior, but something that could be applied to any object with any behavior. So you want to have a bullet that's gravity-based? Throw a bullet behavior on your object and add a Gravity Behavior on top of it.
Custom Controls and Gravity? No problem.
Right now, I don't quite see how you'd get those things without a separate gravity behavior.