Google announced the developer preview of Google Site Kit on GitHub. They’re launching it on GitHub first so this way they allow WordPress developers to test drive the plugin and test Site Kit compatibility with other plugins. For now, it’s the easiest way for Google to gather implementation feedback and recommendations for this plugin.
Google’s current implementation of the plugin setup flow requires developer know-how and familiarity with Google Cloud Platform and OAuth verification process. The current setup experience is not the final user experience they plan to make available to WordPress users. Learn more about it on GitHub.