In the Gmail app you have two options for the swipe left or right gesture. You can choose to have swipe left/right archive an email or delete an email. Here's how to enable the swipe gesture to delete an email instead of archive:
- Open the Gmail app on your iOS device.
- Tap the three lines in the upper left hand corner.
- Tap Settings.
- Select the email you want to edit.
- Tap "When removing messages, I prefer to..."
- Tap Move to trash
Now emails will be moved to your Trash when you swipe on them. You can recover an email by opening the Trash in Settings and selecting the email you want to recover. Tap the three dots in the upper right hand corner, then tap "Move to" and select where you want to move the email (Primary, Social, Promotions).