Taking advantage of Apple's addition of interactive notifications in iOS 8, quick actions enable Gmail users to archive or reply to emails directly from a notification without leaving the app. The functionality also works for notifications in Notification Center and on the Lock screen. The latest release of Gmail for iOS pushes the app to version 4.0 on the App Store.
What's New in Version 4.0Gmail for iOS [Direct Link] is free on the App Store for iPhone and iPad.
• Take quick actions - Archive or reply to messages directly from a notification
• Email files from other apps - Attach photos and documents to Gmail right from your favorite apps
• New attachment viewer - Open attachments from Gmail into any app