I've encountered following error message when I'm trying to import a postgres dump file to Google Cloud SQL.
The input is a PostgreSQL custom-format dump. Use the pg_restore command-line client to restore this dump to a database. This may take a few minutes. While this operation is running, you may continue to view information about the instance.
Please let me know how to import a postgres dump file (which is hosted on Google Cloud Storage) to Google Cloud SQL?
https://cloud.google.com/sql/docs/postgres/import-export/creating-sqldump-csv
and from error you see I assume you used custom format, thus the import sql you prepared won't work (at least by documentation)
https://www.postgresql.org/docs/current/static/app-pgdump.html