Those with Android 4.0 or higher will notice a faster search, even when offline, and there is the usual bug fixes and performance enhancements. For those still rocking a phone using Android 2.2, the update brings improvements to performance and adds new Labels API for third party developers. And we can't forget the new swipable sidebar labels.
The update will eventually show up in the Google Play Store, if not today, than tomorrow. If not tomorrow, than the next day. In other words, this is an update that will be pushed out to the Google Play Store in waves. If you're that impatient, you can sideload the updated app from this link.
GMail for Android update
source: GmailforAndroid, AndroidPolice via Phandroid