Actionable notifications allow you to quickly decide what to do with a message. When you swipe left on a notification, you'll be able to choose either to archive the message or reply to it. Selecting reply will open Gmail and start a reply message for you.

The share extension allows you to share photos and files through Gmail using any app that supports the iOS 8 share sheet. After you activate the extension, selecting Gmail from the share sheet will open a new message pane with your attachment inside, ready to fill out and send.

This update also brings a new attachment viewer, allowing you to open email attachments in any compatible application. You can download Gmail 4.0 for iPhone and iPad from the App Store now.