Mint will be running "Gnome 3", the framework, but not "gnome-shell", the user interface. Unity itself is similarly just a interface atop Gnome 3. Mint will be running its own new environment called "MGSE" (mint gnome-shell extensions), which is basically gnome-shell customized to look and behave like Gnome 2, because that traditional desktop experience is what most current users want and can get neither with Unity nor gnome-shell, which both have basically abandoned their traditional desktop user base and are now betting their future on tablets.
