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PPA
Ubuntu GNOME is not shipped with the latest version of GNOME. However, you can use the latest version of GNOME:
GNOME3 Stable PPA - This is the stable version, and contains updates for any packages that didn't make it into the Ubuntu archives for the current stable cycle.
GNOME3 Staging PPA - staging is for the things that have known regressions and/or bugs. Basically a place to get the bleeding edge packages that are still a work in progress and under testing.
Launchpad
Note
Security and bugfix updates for Ubuntu GNOME are provided by Ubuntu GNOME Developers and Community.