Hi @nywilken, thank you very much for the follow-up.
Yes, I am running from a Google compute instance. I was able to successfully add an SSH key using importsshpublickey method on the instance.
Regarding Oauth2 API, the org I work within does not currently permit this and so there are currently no Oauth clients setup within the GCP service project I am working on.
I have raised a bug report on the Packer GitHub.