You have a managed instance group comprised of preemptible VM ' s.

as the instances (normal or preemptible) would be terminated and relaunched if the health check fails either due to application not configured properly or the instances firewall do not allow health check to happen.

GCP provides health check systems that connect to virtual machine (VM) instances on a configurable, periodic basis. Each connection attempt is called a probe. GCP records the success or failure of each probe.

Health checks and load balancers work together. Based on a configurable number of sequential successful or failed probes, GCP computes an overall health state for each VM in the load balancer. VMs that respond successfully for the configured number of times are considered healthy. VMs that fail to respond successfully for a separate number of times are unhealthy.

GCP uses the overall health state of each VM to determine its eligibility for receiving new requests. In addition to being able to configure probe frequency and health state thresholds, you can configure the criteria that define a successful probe.