Cost-efficient hosting for CI/CD?
Given that you mentioned rarely using some k8s resources, you could explore some alternatives:


With gcloud SDK you could create scripts to be executed for the periods you want to reduce GKE nodes (even to zero) during periods you're not using it
With Google Cloud Functions (GCF) you can create some python/Node.js/Go script to access GKE APIs and also execute the operations to stop/reduce your instances. These GCF could be triggered by schedulers configured by Cloud Scheduler. The cost here for such executions will be really low or even zero.