The default behavior for Gmail accounts is to archive, not delete (in accordance with the Gmail design philosophy). To change it on a per-account basis:
Go to Settings > Mail, Contacts, Calendars and tap the Gmail account.
Tap the "Account" button at the top which shows the email address.
Tap "Advanced" in the popup.
Find "Move discarded messages into:" and select "Deleted mailbox" instead of "Archive mailbox".
Back out to the Account popup and tap Done to save the change.
Now the archive icon will be replaced with the trash can, when you swipe left on a message you'll see the Trash option instead of Archive, and swiping left all the way will delete the message instead of archiving.