During evaluation in 2007, we did a few manual export/imports of calendars & addressbooks. It's far from perfect, but better than data entry from scratch.
Calendar needs to be exported from the GW fat client home view, not the Calendar page. Right-click on Calendar, export as ics. This does *not* include "Notes," just appointments. The ical will often import fine into Zimbra, though GroupWise does very strange things with recurrences.
Sometimes the GroupWise client, and sometimes the server, would crash while exporting as ical. Attempting to export a Proxy calendar would always result in a client crash. We were running GW 7.0.1 on NetWare 6.5.
Addressbook export is a menu item under the Addressbook tab. Export as *both* .TAB and .VCF formats; sometimes one is more reliable than the other. Be aware that some special GroupWise hierarchical organizational constructs will be lost.