Gmail web app supports swipe-to-archive feature. This eliminated the need of separate app for gmail on iPhone. With iPhone 3.0, they’ve added swiping gestures within the iPhone’s browser.
Google’s Gmail blog reads:
After reading the subject of an email and the first line of the message, I often know if I don’t need to open the email to read the rest. With swipe-to-archive, I can simply swipe my finger across the email in the inbox, either from left-to-right or right-to-left, and then tap on the red ‘Archive’ button when it appears. Please note, this feature is only available for the iPhone.