Matt Mullenweg tweeted out a link to Gutenstats.blog this evening, a new site dedicated to tracking Gutenberg beta testing data. The site shows there are currently more than 420,000 active installations of Gutenberg, a slightly more precise number than reported on the WordPress.org plugin page (400K+).
Gutenstats also tracked 213,000 posts written with the new editor and 8,142 posts written yesterday. These numbers were collected from posts made on WordPress.com and Jetpack sites since late August 2018 and a note on the site says the actual number is higher.
In June, Mullenweg unveiled a roadmap for Gutenberg to land in WordPress 5.0. At that time the plugin was active on just 14,000 sites. He proposed 100K+ sites having made 250K+ posts using Gutenberg as a threshold for adequate pre-5.0 testing.
Gutenstats tracking shows testing has far exceeded the original goal for active installations and should reach the posts written goal in just a few days. Mullenweg said they plan to add some block stats to the tracking page in the future.