Is this correct?
1. Users will pull their mail from the internal Zimbra server.
2. Mail from outside your company will will be delivered to the Gmail server.
3. Mail on the Gmail server will be synchronized with Zimbra.
This can be done with one of the many IMAP sync tools out there. See
http://www.emailquestions.com/imap-tools/8081-imap-sync.html for one of them or do a Google search.
What about sending mail? Do you want users' mail to be queued up on their PCs until the Internet link to Gmail is back in service? Or do you want to use Zimbra as a mail relay service to Gmail (storing outbound messages until the link is back)?
If it's the latter I'll have to defer to someone more knowledgeable about setting up a mail relay.
-Rick