Gutenberg is a plugin this year, and only a plugin

The editor is under active development as a feature plugin, which means it can be installed, it changes weekly, and nothing about a site depends on it unless someone chose that.

wp plugin install gutenberg --activate
wp plugin deactivate gutenberg   # and the editor is exactly as it was

Worth installing on a staging copy to see what a custom meta box does under it, because the answer for a good number of plugins is “disappears”. What is not worth doing this year is building against its APIs: the block registration signature has changed several times and there is no stability promise until it reaches core. Classic editor sites are unaffected and will be until 5.0.