New Gmail Update (New slide-out navigation, better typography, & more)
The much-needed update to Gmail is rolling out to Android users over the coming weeks via the new staged rollout features in the developer panel. This is mainly because there are literally millions and millions of people who use this app frequently, so an auto-update to every device all at once would be pretty heavy on Google's servers.
If you don't want to wait for the update from the Play Store, (I didn't) Android Police has already uploaded the new APK to their servers, and you can download it from here.
I've been using it for a little while now. It's a massive improvement!