If you want to migrate Zimbra file to Gmail account then you can try any commercial application like KDETools Zimbra to Gmail Migrator. This application has many advanced features such as:

This application migrate Zimbra file to Gmail, G Suit, Google Drive and etc.

This application shows a preview of Zimbra emails, contacts, calendars, briefcase, and all before conversion.

This application maintain folder Structure during the conversion process.

Read More: https://www.kdetools.com/zimbra/gmail/