The GEL, or Google Experience Launcher that had its debut with the Nexus 5, is now available via the Play Store. Just like other Google apps, the company decided to push the app to the Store to presumably ease the process of updates. They can now update the launcher just like any other app, which is handy.
Something else to note is the name. Across the web, the launcher has been referred to as the Google Experience Launcher. With its debut on Play, the name has become the Google Now Launcher, emphasizing the quick access to Google Now via a single swipe. However, the app is still not compatible (officially) with anything other than Nexus or Google Play Edition devices. This could change in a future update.