From a Home screen, tap Apps (located in the lower-right).
From the Apps tab, tap Gmail.
From the Inbox, swipe the appropriate message(s) left or right.
If deleted in error, immediately tap Undo.
Alternatively, while reading a message tap Delete (located in the upper-right).
To delete several or all messages at once, refer to Delete emails & recover from Trash and click Computer then choose the desired option. This process can't be completed from the mobile app.