From NFO:
The Unreal Tournament 2003 Gateway is a gateway application that intercepts traffic
between the Epic Masterservers and your UT server.
More specifically, the gateway drops packets from the masterserver that was meant
to tell your UT server to disconnect users due to invalid CD-keys.
This can be good if you've misplaced your CD-Key =).
http://elite.to/ut2k3gw or
http://ut2k3gw.da.ru for more info.