I'm not using the gmail app, but was using the built in mail app with it set up through exchange.
The good news is that it looks like you can turn on the "delete means delete" option for GMail Exchange. This rolled out fairly recently, very quietly. And the option is somewhat hidden. On your phone:
- Visit m.google.com/sync and login.
- Select your device from the list of devices (probably only have 1-2). Easy way to know which is right is to look for the last sync time.
- Click on 'Enable "Delete EMail As Trash" for this device.'
This is the same way you enable the ability to send mail as an alias for Google Sync as well.
It's been working well for me. I can move mail to "All Mail" to archive, and delete to delete. It acts just like any other mail account on Exchange, which is really what I want.