So users may subscribe to the gateway presence and the gateway should approve the request subscriptions. The server will then send probe presences to the gateway so make sure that the gateway will respond to the probe presences.
Moreover, the server will send a disco#info IQ packet to the gateway when registering the new component. If the gateway answers the disco request with his identity and supported features then the server will add the gateway to the list of disco#items related to the server. This means that when a disco#items request is sent to the server itself then the answer will contain an item with the gateway JID. So clients may then want to send a disco#info to the gateway itself to discover more information.