When working with Google Container Registry it is recommended that you use JSON keys rather than the short lived access tokens.
JSON key files are long-lived and are tightly scoped to individual projects and resources. You can read more about JSON credentials in Google's documentation at the following URL Google Container Registry advanced authentication
Once a JSON key file has been created with permissions to read from the container registry then the registry should be added with the username _json_key and the password should be the contents of the key file.
In the following example a file named key.json in the current directory contains the JSON key with readonly access to the my-repo repository within the my-project Google Cloud project.
$ anchore-cli registry add us.gcr.io _json_key "$(cat key.json)"