Hi,
If you want to see Zimbra-Google Calendar CalDAV integration, please vote for this Request For Enhancement (RFE):
Bug 39402 – CalDAV functionality with Google Calendar
P.S. gnobush, if you're still looking into creating a Zimlet and have ideas of how to achieve it, I'm willing to assist. (A disclaimer, though: I've never written a Zimlet before.) If so, PM me.