Your application images are built and pushed to Google Container Registry (GCR).

https://cloud.google.com/architecture/continuous-delivery-toolchain-spinnaker-cloud

https://spinnaker.io/guides/user/pipeline/triggers/pubsub/

 The most efficient way to build an automated pipeline that deploys the application when the image is updated is to use Cloud Pub/Sub to trigger a Spinnaker pipeline. This way, you can leverage the built-in integration between GCR and Cloud Pub/Sub, and use Spinnaker as a continuous delivery platform for deploying your application .