RFC5546 specifies that participant status (accepted, tentative,
declined) should be sent to the organizer of the event. That
organizer is not necessarily the sender of the invitation; for
instance Google Calendar uses custom email addresses to receive these
notifications.
* lisp/gnus/gnus-icalendar.el (gnus-icalendar-send-buffer-by-mail):
Replace the default recipient of the reply by the organizer of the
event.
(gnus-icalendar-reply) Pass that organizer to the previous function.