[kkfx]

There is https://github.com/emacs-mirror/emacs/blob/master/lisp/calen... and https://github.com/dengste/org-caldav but that not much related to Gnus approach of "unifying messages in a single UI", it could be used to import from ics files to org-agenda or creating an ics file or sync org-agenda with Radicale/Baïkal/Davis CalDAV servers