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.
Other desktop environments install other background services. The Whonix ™ project hasn't researched the security/privacy implications of having these installed. For example, GNOME automatically installs GeoClue. (systemcheck warns if that very package is installed, but still... Other desktop environments also ship a different selection of default applications that have not necessarily been checked for security/privacy. Installing your own desktop environment may result in installing software packages not well hardened or chosen based on security wise.
Since Whonix ™ is an Open Source / Freedom Software project, Whonix ™ developers are hoping that other developers join the project and maintain other desktop environments. That someone could be you?