The goal Matt set for Gutenberg at WordCamp Europe was 100,000 active sites and 250,000 posts, although where those totals should come from wasn't defined. It now appears that Jetpack and WordPress.com are the focus, and if that's the case, then currently the goal for sites has been surpassed. It's not quite there yet for posts, however.
Matt Mullenweg reported on Thursday (September 20) that Gutenberg has over 420,000 active installations, and 213,000 posts written with Gutenberg have been tracked. Note that this is only for Jetpack and WordPress.com sites. The total number would be higher if self-hosted installs that are not using Jetpack were included. A new blog (gutenstats.blog) has been setup to showcase stats like these. It appears to be updated often, perhaps even daily. Block stats will be coming soon.