http://forum.blockland.us/index.php?topic=203574.0
You want map-based gravity? Then if you download this: http://orbs.daprogs.com/rtb/forum.returntoblockland.com/dlm/viewFile3d7c.html?id=4479 along with the required mod it states then you can do that. If you want zones that give gravity when other zones do not, then it'd take a bit more to code.