You can try Zimbra to Gmail Converter application that simply migrate Zimbra files to Gmail without losing data. The application can restore and extract different file formats of Mail, Contacts, Calendar, Tasks, Diary, etc.
First of all, download Zimbra to Gmail Migration Tool and choose one option from Add Files or Add Folders. Choose Zimbra TGZ files from your Windows PC and upload them to the software panel. Now you can preview Zimbra emails, contacts, calendars, and other important details in the software panel.