I think it may be a good idea to create a support ticket for this; I would like to review your WinGate settings because NAT should allow your XBox 360 to have complete connectivity.
http://support.qbik.com/index.php?_a=tickets&_m=submit
1. WinGate Registry.
GateKeeper --> Options menu --> Advanced --> Save Registry
2. WinGate Config Report
GateKeeper --> Options menu --> Advanced --> Save Config Report
3. ipconfig/all from the WinGate Server
(Windows) Start menu --> Run --> cmd --> ipconfig/all >> C:\ipa.txt
4. IP Address, Subnet Mask, Default Gateway and DNS address of the XBox.
5. Elaboration on this comment, and any links regarding and port numbers:
This sounds very much like the problems reported with moderate/strict NATs over XBox Live, but the console itself reports an open configuration... =S