Meld is a visual diff and merge tool for the GNOME desktop. Meld lets the user compare multiple files and merging updates/edits in-place. GNOME's Meld also supports working with Git, Bazaar, Mercurial, Subversion, and CVS version control repositories.
With the release just moments ago of Meld 3.11.0, this GNOME application was finally ported to GTK+ 3, GObject Introspection, GApplication, GSettings, and other GNOME 3 technologies. The application now has CSS styling, support for more modern GTK widgets, and other new features mostly on the user-interface level.
More details on GNOME Meld 3.11.0 can be found via this mailing list post.