I switched to K9 but I just checked my Gmail app. Mine had check boxes on the left of the screen which I can tap and then press delete. Look at your settings(tap menu, tap settings) and see if batch operations is checked.
I can check off multiple ones and delete them all at once but can not check them all off at once, which is what I think you're looking to do. You can check off a full page worth of them via the web interface though. While it would be nice to do it all in one app if it is a big pain you could just delete them from a shortcut on the phone's browser or a computer later. I'll browse my emails on the phone but tend to reply from the pc so I'm not an exclusive phone email user.