Other Desktop Environments
The Downloadable Whonix ™ versions are available with either:
However, other desktop environments like Gnome, KDE, LXDE and so on can be manually installed.
When installing your own desktop environment such as GNOME, you're on your own. If you are going that path, start with Whonix ™ CLI, the text terminal only version of Whonix, which comes without a desktop environment installed by default, and install a desktop environment of your choice there. This is much better than starting with Whonix ™ XFCE and then uninstalling XFCE to install another desktop environment.
In theory (also tested a little bit), everything should work except Whonix ™ open-link-confirmation [archive] (asking for confirmation, if you want to open a link you click outside the browser in Tor Browser to avoid linking) and Tor Browser won't be the default browser by default. 
Other desktop environments install other background services. We haven't researched the privacy implications of having these installed. For example, Gnome automatically installs GeoClue [archive]. (Whonix ™ prevents installation of that very package, but still...) Other desktop environments also ship a different selection of default applications that have not necessarily been checked for privacy. For example, Thunderbird is the only recommended mail client (as explained on the E-Mail and Encrypted Email with Thunderbird page). Installing your own desktop environment may result in installing software packages such as KMail, which is discouraged to use in context of Whonix ™, since there is no TorBirdy-like extension for KMail.
Since Whonix ™ is an Open Source / Freedom Software [archive] project, Whonix ™ developers are hoping that other developers join the project and maintain other desktop environments. That someone could be you?