When I go to http://pkg-gnome.alioth.debian.org/d...86/Packages.gz
I get the error:
The requested URL /debian/dists/experimental/main//binary-i386/Packages.gz was not found on this server.
So the URL is invalid.
Many people have had problems upgrading (k)ubuntu 7.04 to 7.10 and beyond (8.04 is the most recent).
A fresh install is recommended, and you'd be better off going for 8.04 as 7.10 is no longer supported.