This read-only IpAddress is the next-hop gateway address toward the origin subnetwork for reverse-path forwarding, reported as 0.0.0.0 for directly-connected routes. An admin reads this to trace the upstream path multicast traffic from a given source will be reverse-path forwarded through. Seeing sipmRouteGateway as 0.0.0.0 tells the admin the origin subnet is directly attached to this device rather than reached via another router.