Folks who use Gmail through Mail app on iPhone, iPod touch or iPad have observed that whenever you want to delete any of your email, it become an archive instead of going to Trash folder in Gmail. To resolve the issue Gmail adds new feature of “Delete” through Google Sync on iPhone, iPad or iPod Touch.
Check also:
How to Change Default Outgoing Mail Account on iPhone
How to Setup Gmail on iPhone
How to Setup Gmail in Apple Mail
Through guide below you will learn how to configure Google Sync to delete messages on iDevice while you have setup your Gmail account as a Microsoft Exchange account.
Delete Gmail Messages on iPhone, iPad or iPod Touch
Step-1
Visit m.google.com/sync through browser on your iDevice and Sign in with your Google Account
Step-2
Tap on your iDevice
Step-3
Tap on option Enable “Delete Email as Trash” for this device and that’s it