The Halo rings get their "gravity" through centripetal force. As the ring spins, centripetal force pushes all spinning bodies away from the center of rotation. The ring is uniform, and because of this all points are the same distance from the center, therefore centripetal force applies evenly. This force also pushes objects on the surface of the ring away from the center of rotation, or pushes them down on the surface of the ring, translating into gravity. Mountains, forests and waterfall were created by Forerunners.