Path from .git/config in origin tag something like https://source.developers.google.com/p/PROJECT/r/REPO
Set the env var like this:
go env -w GOPRIVATE=source.developers.google.com/p/<PROJECT>/r/\*
Then, go get will work, note .git extension required to work directly over git, this then uses the gcloud credential helper configured in the .git/config inside this repo,