code: func (gce *Cloud) RemoveInstancesFromInstanceGroup(name string, zone string, instanceNames []string) error {
  	if len(instanceNames) == 0 {
  		return nil
  	}
  	instances := []*compute.InstanceReference{}
  	for _, ins := range instanceNames {
  		instanceLink := makeHostURL(gce.projectID, zone, ins)
  		instances = append(instances, &compute.InstanceReference{Instance: instanceLink})
  	}
  	op, err := gce.service.InstanceGroups.RemoveInstances(
  		gce.projectID, zone, name,
  		&compute.InstanceGroupsRemoveInstancesRequest{
  			Instances: instances,
  		}).Do()
  
  	if err != nil {
  		if isHTTPErrorCode(err, http.StatusNotFound) {
  			return nil
  		}
  		return err
  	}
  	return gce.waitForZoneOp(op, zone)
  }
docs: // RemoveInstancesFromInstanceGroup removes the given instances from the instance group.