As I said, you can fake it by actually rotating the room the player is in. So after the rotation the player walks on the ceiling. And the rest of the level is made flipped so it will look like the gravity changed for the whole level.
While really just a small part was rotated...
You see, it's a way to fake it. Ever seen Gunman: Chronicles intro? Apart from that it looked horrible, the effect with the elevator and the seemingly different gravities was done the same way.