The two gateway-site.xml properties associated with the new Ambari cluster monitor need to be added to the table in the Gateway Server Configuration section of the docs.
gateway.cluster.config.monitor.ambari.enabled | Indicates whether the cluster monitoring and associated dynamic topology updating is enabled. | false
gateway.cluster.config.monitor.ambari.interval | The interval (in seconds) at which the cluster monitor will poll Ambari for cluster configuration changes. | 60