Hello,
This is a discussion group for the Cloud SQL product, which is not affiliated to pgAdmin. For that reason, the support you would get for permission issue your are facing with pgAdmin would be limited here, and this would be better addressed on StackOverflow in the future.
Since that way to modify permissions is for Windows, you would need to do so using Mac's Terminal.
Alternatively, I believe that this
article might interest you, as it mentions the use of psql's \copy, just like the error message, and the pgAdmin GUI. For psql, the command would be `$
\copy table_name to 'filename.csv' csv header` as per this
link.