List media-gateway on CM is generally my go to command. If not registered, can you ping the gateway? If you are able to ping and stable, then check to see if the ESS is active and if the gateway is registered on it. One of two ways, log in to the ESS and execute command: reset system 4 or login to gateway and execute command: reset chassis. If gateway is in pending state, login to gateway and execute command: reset chassis.