Migrate all Compute Engine instances to Preemptible VMs and use the Always Free tier for Cloud Storage, Cloud SQL, and Cloud Pub/Sub.

Utilize the GCP Cost Calculator to estimate costs, then create a custom machine type for each Compute Engine instance, use Nearline storage for all Cloud Storage objects, and disable all Cloud SQL instances and Cloud Pub/Sub topics.

Enable autoscaling and use managed instance groups for Compute Engine instances, optimize Cloud Storage by leveraging object lifecycle policies, employ Cloud SQL with automatic storage increase, and monitor Cloud Pub/Sub usage to adjust quotas.

Move the entire application to a single Compute Engine instance with maximum vCPUs and memory, and use the Always Free tier for Cloud Storage, Cloud SQL, and Cloud Pub/Sub.