Starting in GP Premium 1.6, the Import/Export module has been re-written from the ground up. It now allows you to export all of your GP options into one file, instead of one for each individual module.
You can still choose to only export individual modules if need be.
NOTE: Customizer > Additional CSS is not part of the themes import/export settings. This should be manually copied and pasted between sites.
Exporting your Settings
Importing your Settings