In the earlier version of Gmail for iOS, once you booted the app you had to wait for it to refresh the inbox and fetch the new messages. This was quite annoying and could take quite a while in some cases. With the new update, Google has effectively removed the necessity to wait.
Apparently, the new ‘background app refresh‘ feature in the new Gmail for iOS app makes sure that all new messages in your inbox are ‘pre-fetched and synced so they’re right there when you open the app.’ However, the new feature works only on iOS 7 handsets. If your device run iOS 7, you need to ‘turn on background app refresh and notifications (badge or any other type) for the Gmail app.’
The app also brings some other minor improvements. According to the official blog post, “The Gmail app also now supports sign-in across Google iOS apps, including Maps, Drive, YouTube and Chrome. Sign in to one, and you’ll be signed in to all (this also works for signing out). So you won’t have to type in that 27-character password or retrieve your 2-step verification code every time you navigate to another Google app. You may need to re-login after you update the app, but then you’ll be all set.”
You can snag the new, updated Gmail for iOS app here.