TGZ is an unextracted data file that the user cannot open any other email client without conversion. Also if you try the manual procedure to convert and open data in Outlook and any email client then you may fail.
Here this-party solution makes this conversion task easier for everyone. MailsDaddy Zimbra Converter Tool is the most effective GUI application tool for loading and exporting TGZ files in PST format. Launch the tool and click on "Add File" to select your TGZ files. Preview is optional, so click "Export", then choose PST as the output format. Specify a destination folder for the converted file and click “Next” to start the process.
For a free demo edition, the software exports up to 20 emails from each folder. Read more; https://www.mailsdaddy.com/zimbra-converter/