Going in one side of the portal sets gravity to 0.2 (normal is 1). Going back through the other side of the portal sets gravity back to normal.
Normally you'd put this "portal" in a doorway (or all doorways to a room) so that it would appear that the "room" is the only place where gravity changes. But you don't have to!
Uses: trigger_gravity
I realized this is an example map, but you could trigger sound and/point light when you cross the gravity threshold to spruce it up a bit perhaps.
Nice job!