Google continues to use elements of its cards system, which is how information is received using Google Now. It apparently has been Mountain View's goal to use the cards to make as many Google apps as possible look the same.
If you have your Android handset configured for automatic updates, you can just sit back and wait for the notification. If you don't have your phone enabled for that feature, you can check into the Google Play Store every now and then, or you can go into the settings and make the change to receive automatic updates. And if you're worried about your monthly data plan getting consumed during one day with automatic updates, no worries. You can set your phone to automatically download the updates only when you have a Wi-Fi connection. As you see from the screenshots below, updating to the new Gmail UI should be a priority for those who use the application often.
source: +Gmail via AndroidCentral