You can export the full GMail archive of a user in either ZIP format or the PST format. In the user's archive, select the GMail archive. In the GMail archive page you will see 2 export options, 'Export' & 'Export to PST'(refer pic1). The 'Export' option will export you data in ZIP format. The 'Export to PST' option will export the data in PST format.
There are 3 sub-options for Export(refer pic2),
  1. All - Exports all the mails present in the GMail archive of the user.
  2. Selected Label - Exports only the mails present in the Mail folder/label selected in the left pane by the user.
  3. Selected Item - Exports only the particular mails selected by the user in the archive.

Once Export is initiated, you can view the progress of the export in the Backup> Reports> Export Report page(refer pic3). Once the export completes, a download link will be available in the same page from which you can download the exported data to your local drive.