Putting this line in your .gnus.el should work: (setq mail-sources '((file :path "/var/mail/userid")))
Replace userid with your own login, of course. In principle, just (setq mail-sources '((file))) should also work. Check the "Fetching mail" section in the info manual for gnus. -- Eric S Fraga via gnus (Emacs 30.0.50 2024-02-26) on Debian 12.0