Gutenberg is a ReactJS based WordPress project with a focus on the editor in the core. Gutenberg brings a completely new editing experience to WordPress for media-rich pages and posts.
Gutenberg is built on ReactJS. I am a member of the core Gutenberg development team and I contribute to it as an open-source developer. As of now, I have over 30 contributions to the project and counting.
An introduction to the Gutenberg project can be found here.