Starting with iOS 4, the Mail app started archiving instead of deleting Gmail messages. This can be a little confusing because it's not immediately clear where this "archive" is.
To see your archived Gmail messages:
- Open Mail
- Tap on Mailboxes on the top left of the screen
- Tap on All Mail under the Gmail folder to see all your messages, whether they are archived or not.
- Open Settings> Mail, Contacts, Calendars.
- Tap on Gmail account.
- Swipe Archive Messages to OFF.