Hi Mikael,
The gd:recurrence element includes content as specified by the
iCalendar standard (RFC 2445). According to that standard, a TIMEZONE
definition is required.
From the RFC (http://www.ietf.org/rfc/rfc2445.txt):
An individual "VTIMEZONE" calendar component MUST be specified for
each unique "TZID" parameter value specified in the iCalendar object.
I guess the moral of the story is -- supply the VTIMEZONE to work
according to spec, but it currently does work without it :)
Hope this helps!
Happy coding,
-Ryan