In some gcloud code I'm finding a comment:
Stored "well known" file associated with
gcloud
command line tool
What is this "well known" file?
I believe this is what you're looking for: https://github.com/google/oauth2client/blob/master/oauth2client/client.py#L1367
For Linux/MacOSX, it's ~/.config/gcloud/application_default_credentials.json