If set up correctly and you do have a Google drive account then you should be able to export but I haven't tried this recently and I know Google have changed some things recently (I will check) That documentation page is not meant to have the instructions as it is part of a set of pages which just defines the "capabilities" of Moodle. Here is the page for Google Drive portfolio with an example as an html file.
General help
How do I make it easy to download their submissions?
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