Other Desktop Environments
Downloadable Whonix versions come with KDE installed by default. It is possible to uninstall KDE, although doing so is a bit difficult, especially if you want to uninstall all of it and/or to use a CLI version and/or to install their own desktop environment such as Gnome, LXDE, etc. See Whonix Debian Packages.
Users who build Whonix from source code using Dev/Build Documentation can use an optional Terminal-Only build configuration option. And then decide from there, if they want to install a desktop environment and which one.
When installing your own desktop environment, you're on your own. In theory (also tested a little bit), everything should work except Whonix's open-link-confirmation (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. (Whonix prevents installation of that very package, but still...) And comes with a different collection of default applications that have not necessarily been checked for privacy. For example, Thunderbird/TorBirdy (as explained on the E-Mail page) is the only recommended mail client. Installing your own desktop environment may result in installing KMail, which is not recommended, since there is no TorBirdy-like extension for KMail.
At the moment unfortunately, "Whonix should support other desktop environments" is a synonym for "Patrick should support other desktop environments". Due to time constrains it won't be possible for Whonix developer Patrick Schleizer to maintain a second desktop environment next to maintaining the KDE desktop environment.
Since Whonix is an Open Source / Free Software project, Whonix developers are hoping that other developers join the project and maintain other desktop environments. That someone could be you?
- The tb-default-browser package has not been made to work / tested with Gnome yet.
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