Google unveiled a brand new Gmail web app back in April. It has Google's new Material Design theme, deeper integration with Calendar/Tasks/Keep, quick actions, snooze, and more. Organizations using G Suite could opt in to the new Gmail since day one, but customers of the legacy Google Apps plans couldn't try it out.
At long last, the revamped web app is now rolling out to Google Apps users. It's unclear at this point when Google Apps customers will be forced onto the new layout, as G Suite accounts will be in October.