You'll be able to export your calendar from Zimbra and import it right into Gmail [1]. You will need to re-share your calendar with those who previously had access. See Moving your Zimbra calendar to Google Apps [2]
Links:
[1] http://www.humboldt.edu/its/glossary/5#term195
[2] http://www.humboldt.edu/its/google-move-zimbra-calendar