I’m fairly certain that this is caused by a client that is running out there that is currently in the “Attempting to re-connect to Gateway” mode.
You won’t see this client on the sessions list because it doesn’t have a session. Each time it tries to reconnect it attempts to re-authenticate, but apparently the user/password combination it has is no longer valid.
Unfortunately, restarting the gateway won’t help (the client will still be out there, somewhere…)
You might be able to use netstat or wireshark to see where incoming traffic is originating from on the gateway’s HTTP port. Or you could change the port to a different port so that this client couldn’t connect at all. Or you could find the client and shut it down 