A limited frontend preview of the Gutenberg editor, not to be confused with a frontend editor. It's literally the Gutenberg block editor post interface, but loaded on the frontend. It also includes some filters to prevent logged out users abusing APIs, but allowing Gutenberg to still function
Also, be careful, if you're logged in it will be fully functional, other users will see autosave notices and your changes will be saved!
It's a WP theme, upload and activate and make sure you have the Gutenberg plugin installed and activated first.