Google updated the mobile web version of Gmail and tweaked the UI to make it look more like the iOS version. The Gmail app for the iPhone and iPad was updated back in December 2012 with a fresh, new UI that now is duplicated in the web version. Besides the new look and feel, the web app also has a better search feature and improved integration with Google Calendar.
You can see the changes by logging into gmail.com using the web browser on your mobile device.
[Via Google]