I know that theme devs that have/add themes to .org are starting to update their own there. I know with Blogging Theme Styles, the site already has the last few (new) themes fully supportive with block styling. So it seems others are getting onboard. The Hemmingway theme recently updated to support Gutenberg as well.

Not sure how many existing themes in .org will update, but it appears Automattic will be doing this for all their themes.

But one thing I've noticed is that the Gutenberg team seems to like making changes to styles, which means we now have to back in and update the style classes. An example is the Cover Image block. Used to be "wp-block-cover-image" now its "wp-block-cover", so this broke styles.

A challenge because Gutenberg is not officially released, or finished.