Have a look on this website
http://www.pctools.com/guides/registry/detail/269/You could export your current registry settings for the adaptor you use, then change the value for the default gateway and save the file on everyone's desktop. Then if the gateway goes down, you can ask them to run the file and they should be back online.
The users will need to have admin rights to the machine though, or at least that reg key, as it's under Local Machine.
We had a similar plan in the event of our gateway going down whereby we could route traffic through another buildings broadband connection.
Hope this helps?
Tom