code: func (gce *Cloud) SetProxyForGlobalForwardingRule(fw *compute.ForwardingRule, targetProxyLink string) error {
  	op, err := gce.service.GlobalForwardingRules.SetTarget(gce.projectID, fw.Name, &compute.TargetReference{Target: targetProxyLink}).Do()
  	if err != nil {
  		return err
  	}
  	return gce.waitForGlobalOp(op)
  }
docs: // SetProxyForGlobalForwardingRule links the given TargetHttp(s)Proxy with the given GlobalForwardingRule.
  // targetProxyLink is the SelfLink of a TargetHttp(s)Proxy.