The app features an interface optimized for iOS, push notifications, comprehensive search across the whole inbox, threaded conversations, one-click photo uploads, and split view (only on the iPad).
Unfortunately, mere hours after its launch via iTunes, the Gmail iOS app has been removed by Google because “it contained a bug which broke notifications and caused users to see an error message when first opening the app.” Google is currently working to fix the problem and re-release the app. Meanwhile, “everyone who's already installed the app can continue to use it.”
When the bug is fixed and the Gmail app is again available in the App Store here, it will be compatible only with devices running iOS 4+.