Now that the shiny new Ubuntu 9.04 is available, the servers with packages for 7.10 are closed down. Ok, how to upgrade a server, if you can't even install the "update-manager-core" package - because of the missing repositories?
(Backup and) edit your /etc/apt/sources.list file, comment out any third-party software sources. Replace every occurence of the old distribution name ("gutsy" in question) with the new name ("hardy" for the latest Long-Time-Support release, or a newer release). Now start a screen in case your network connection breaks during the upgrade.
aptitude update (make sure that you don't see any 404 errors here)
aptitude dist-upgrade (that's the old name, the new one is full-upgrade)
Reboot your box and voila you should have an upgraded system.