Luckily the recent update to pg_dumpall works (without any extra effort) to dump Globals off the recently launched Google Cloud PostgreSQL service well.
This is because this Postgres service seems to be using the same limitation (as AWS RDS Postgres does) in that none of the user accounts are SuperUser in the Postgres sense. Therefore, this platform too disallows accessing pg_authid.
The fallback here too, is to use pg_roles to read (as much as is possible) to dump Role related Globals. Read here for more info on the recent changes to pg_dumpall that makes this work.