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.