As Gnus documentation suggests, pressing B in the Summary Plugged mode deletes an email. So Gnus doesn't display them, but they're still visible in Web-based gmail client. Please note that I did set Auto-Expunge off - Wait for the client to update the server. and Immediately delete the message forever in my gmail account settings.
Is there a way to fix this? Thanks.
Here is the code from https://github.com/redguardtoo/mastering-emacs-in-one-year-guide/blob/master/gnus-guide-en.org
;; @see http://www.gnu.org/software/emacs/manual/html_node/gnus/Expiring-Mail.html
;; press 'E' to expire email
(nnmail-expiry-target "nnimap+gmail:[Gmail]/Trash")
The mail will be moved to Trash folder.
(setq nnmail-expiry-target "nnimap+gmail:[Gmail]/Trash") since otherwise Gnus was complaining about the error. Now, pressing E marks email as expired. In order to move it in Trash right away, I also (setq nnmail-expiry-wait "immediate"). Does this look right to you?