Back in May 2007, I took a look at Kompare, a graphical utility for KDE which allows you to compare two files side-by-side, a bit like a graphical version of the diff tool.
A similar application for GNOME is Meld.
As you can see if you compare it with Kompare, the interface is quite similar. Any additions made to the original are highlighted in a green block. Deletions and differences between the two files are by default done with a blue highlight and red text colour.
Personally I think I prefer Kompare in terms of how the colour scheme works, as I think it makes it more clear, but nevertheless, Meld is a worthy GNOME counterpart for the application.
True to GNOME style and in contrast to the KDE app, the application is kept nice and simple. The Preferences window does allow you however to configure fonts, the displaying of changes, filters, encoding and support for pulling files from CVS.
All in all, Meld is a nice simple app that does its job well. If you're not too much a fan of diff's command line output and the format that it is in, and you want to see the differences between two files in a more graphical way, Meld works.