Great question — answering as the UphillTech team:
If I recall correctly, when installing a fresh FreeBSD system you can choose to install Xwindows and various desktop managers (gnome and kde spring to mind first). You could also install either via the ports or packages system. More information about installing Gnome-desktop is available at http://www.freebsd.org/gnome/docs/faq2.html#q1
Full answer by dracoling: https://superuser.stackexchange.com/a/297845 (CC BY-SA 3.0).
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