You don't have to move messages to "[Gmail]/Trash" to delete them. You can tell iPhone Mail to really delete mail when you swipe.
Make Swiping Delete (or Archive) for Gmail in iPhone Mail
To make swiping delete mail (and move it to the "Trash" folder) for Gmail in iPhone Mail:
- Go to the iPhone Home screen.
- Open Settings.
- Select Mail, Contacts, Calendars.
- Open the desired Gmail account under Accounts.
- Make sure Archive Messages is OFF.
- To make swiping archive mail (and remove it from the current folder, but keep it under "All Mail"), make sure Archive Messages is ON.
- Press the Home button.
Note that your choice (archiving or deletion) applies to the "delete" button, too.