Google Cloud SDK is required, please install it by following instructions mentioned here for your Operating System. The SDK provides tools (like gcloud) which are used. You can inventory all images in your GCR repository by specifying repository URL or single image by specifying fully qualified image name (with tag / digest).
You can run twigs to ingest this collected inventory into your ThreatWorx instance by following the below mentioned steps:
Open a new shell / terminal
Check that twigs is installed and running properly by running below command:
twigs gcr -h
Sign in into your Google Cloud Platform instance using gcloud CLI as described here on the box where you will be running twigs.