Currently what Gutenberg does is to enable blocks inside the post content. In other words, the only thing adds to theme development is how posts look and behave. All other things are untouched—the post title, the excerpt, the meta, archives, the site header, the sidebar, the widgets etc.
You can’t use Gutenberg to build your entire site from logo to layout. In other words, you can’t let the user to customize the entire site as they are able to customize the content of a post.
Gutenberg deals only with a small fragment of what goes into a theme. Major pain points like customisation still have to go the good old way. If you think Gutenberg is the cure of all what’s hard with WordPress you still have to wait a little bit. I imagine a day when everything building up a WordPress site can be edited, and styled as the post content today with Gutenberg.