Currently, Avaya Gateways cannot be added as VoIP Gateways. Only Cisco gateways can be added. So even when the CLI credentials are correct, it cannot be added as it is not supported.
As part of Avaya Communications Manager polling, the Gateways attached to the Avaya CM is polled. The gateway details can be accessed from the VoIP Call Signaling resource under Call details page (only if the calls passed through the gateway). From the gateway details page, you can view the passed/failed calls chart for the gateway. (As Michal mentioned early)