iOS: Understanding Gmail Archive
This article describes the difference between archiving and deleting a Gmail message, and how to enable or disable this feature on iPhone, iPad, and iPod touch.
iPad, iPhone, iPod touch
Mail archives Gmail messages rather than deleting the messages:
When you archive a message, it moves to the All Mail folder. It does not delete it from the Gmail account. You can learn more about the Gmail archive feature in Gmail Help.
Archive replaces all delete options when using a Gmail account, including using the Trash icon while viewing a message and selecting Edit to manage multiple messages.
You can disable this feature in Settings > Mail, Contacts, Calendars > Gmail Account > Archive Messages: