What does gs protocol mean?
gs://
scheme is used for identifying resources stored in Google Cloud Storage.
You can access the file that you mention as follows:
https://storage.googleapis.com/cloud-samples-tests/speech/brooklyn.flac
So, if you create/upload your own file in Google Storage -- like I'm doing now while testing the same API you are using-- the "equivalence" is the following:
https://storage.googleapis.com/
translates to
gs://
and viceversa.
I have no idea why Google doesn't explain this clearly.
I hope this helps.