With Gmail IMAP, you can either archive deleted messages to the 'All Mail' folder or you can actually move them to the trash folder. The default on iOS 7 is to archive messages. In previous versions of iOS, this setting was available right on the main email account screen. In iOS 7, it's buried a bit deeper. Go to Settings > Mail, Contacts, and Calendars > Tap on your Gmail (or Google Apps) account. You'll then want to tap on 'Account' and then 'Advanced'. You should be seeing something similar to the screen shot below.