As you might remember, Gmail had originally released the app a few weeks ago, but the app contained many bugs. In particular it was unable to receive push notifications to let you know when you received a new email. For this reason, Google pulled the app from the App Store, and had to update it to resolve the problem.
Gmail for iOS allows you to take better advantage of your Gmail email account in iOS. It’s very limited, as it’s based on the version already available online, and only allows you to link it to one account at a time. Also Push Notifications aren’t really what you would expect, as all you get is a badge on the app’s icon and maybe a sound, no actual messages. However, if you like Gmail’s labels feature, and amazing search, it will probably suit you better than the iOS Mail app.
A word of warning however: you shouldn’t update the app just yet if you had previously installed it. Google recommends that you first uninstall the old version of the app, or at least log out of your account before installing this new version. Otherwise, it might not work properly.
As for this update, it doesn’t seem to bring any new features to the app.