I noticed recently when one of our sbs 2008 servers got rebooted the gateway would disappear, the first time it happened i thought nothing of it I just ran the fix my network wizard and that sorted it but the next time it rebooted it disappeared again, it turns out its a known issue i found the answer on the SBS Blog
1. To fix the issue go to the TCP/IP properties of the network card and enter in the proper default gateway.
2. Download the hotfix and install to stop it happening again
3. Manually edit the registry to remove the extra spaces in the following location HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Para
meters\Interfaces\<GUID>\DefaultGateway
The guid is the interface for the network card if you have one there will be one entry if you have two network cards there will be two entries.
3. Double click the value for DefaultGateway and remove the blank lines above and below the gateway address and click ok.
4. Reboot server if needed.