Although I have read in your comments that you are already satisfied - the only real solution for your explicit question "How do I install the official Ubuntu GNOME distribution over an existing Ubuntu?" is that you have to perform a clean installation of Ubuntu GNOME, because this is the only way to get the official distribution.
Adding a "3rd party repository" and installing the GNOME desktop just adds another desktop environment to those ones you already have installed - not less, not more. So, to replace your current edition with Ubuntu GNOME you need to download the ISO file, create an installation media and install it to your disk. Here you find all available ISO files : Ubuntu GNOME releases
You can get close to Ubuntu GNOME by installing the meta-package ubuntu-gnome-desktop
, which includes everything installed by Ubuntu GNOME and then uninstall your current desktop environment, but well, that is quite a lot of work where many things might go wrong ...
again, this answers the original question how to install the official Ubuntu GNOME distribution.
Update and some clarification - because now the question was changed to how do I convert ...
It seems to be a matter of wording, so : you can not convert one distribution to another, but you can add and remove desktop environments to an existing distribution.
Conclusion : It is up to you to decide - if you want to just use the GNOME DE, you can install it, if you want to have the official Ubuntu GNOME distribution, you have to install Ubuntu GNOME.
Please bear in mind that working with different desktop environments needs some advanced knowledge how to fix possible issues such as that one you described in your other question. :)
sudo apt-get install ubuntu-gnome-desktop
and eventuallysudo apt-get install ubuntu-gnome-default-settings ubuntu-gnome-wallpapers
E: Unable to locate package ubuntu-gnome-desktop
apt-get update
is failing too. BTW, you should post that as an answer!ubuntu-gnome-desktop
is a metapackage and, according to apt-cache:Section: universe/metapackages
, be inuniverse
. I'm talking about Xenial here; don't know about older versions.