In particular, working with the gsuite / workspace APIs can be a bit of a challenge.
But in practical terms, you likely want to either setup auth with OIDC (generally preferred / safest) and use that to generate a credential file (you can google "gcp circleci oidc if you want to find examples of that), or create a service account and use a JSON credential. Getting the appropriate permissions from a Google Cloud service account mapped to Google Workspace may be a little tricky. You’re probably going to need a separate project and some permissions to handle that.
I would try to get whatever you’re trying to do working locally first. In both cases, reading this article may be a good idea if you haven’t read it before:
Maybe also take a look at this article
If gcs is a viable alternative, you can use gsutil to copy files, which there are probably some easier / better examples for.