If you need to migrate Google Apps to Zimbra here is a simple howto. Just follow the steps and you will have your Google Apps mails transferred to your Zimbra mailbox.
Accomplish Zimbra Desktop to Google Apps Migration in Easy Process
Zimbra to Exchange Converter to Migrate Bulk Zimbra to Exchange Online
Connect Zimbra Community with Outlook 2013 - VIONBLOG
Zimbra Migration – Convert Zimbra On-Premise
Migrate Google Apps to Zimbra - VIONBLOG
Zimbra to Zimbra Migration - Zimbra :: Tech Center
Zimbra external account Gmail - Zimbra :: Tech Center
Migrate emails from Gmail / Google Gsuite to Zimbra
Migrate Emails from Google Gmail or G Suite to Zimbra Collaboration Server
How to Migrate Zimbra to Google Apps ? Complete Solution
Zimbra to Zimbra Migration - Zimbra :: Tech Center
How to Migrate Thunderbird to Zimbra Client?
How to Migrate Thunderbird to Zimbra Client?