The Gatsby homepage mentions Wordpress as a possible data source. I don't evens see mention of their CMS on the front page. The only "service" I see them advertising there is Preview - which seems to be a development collaboration tool. I don't think they're convincing people to move away from Wordpress.
It's funny that the article talks about moving away from your monolithic "CMS system" (redundant?) so that you can use headless Wordpress or Drupal. In other words, you get to toss your monolithic content management system system so that you can keep your monolithic content management system system.