Finally. After a long time, Google has finally added support for push notifications in the Gmail API.
Until now, if an application wanted to add support for Gmail, it would have to resort to polling the service after set intervals to see if there were any new changes. Polling is a bad idea on mobile, as it consumes a lot more power than push notifications, which are pushed directly to the device without it constantly having to check for changes.
The updated API finally brings this functionality, so developers can now update their apps and include this functionality. It will also be easier and cheaper for developers to have this functionality without resorting to using private servers to push emails to you.