The big addition to the app is the ability to now swipe to navigate to the previous and next messages in whatever folder you are viewing. So, rather than having to back out to the inbox or whatever folder you're viewing, you can just swipe left or right to switch conversations. The update also adds better ways to take action on multiple messages. Just select a message and it'll trigger a new "edit mode", which will allow you to select more messages, and choose an action.
If you want to get the update, just head over to the iTunes App Store.
source: Gmail Blog