How to get GooglePlayGames Current user's id?

Hear is my code;

 void Start () {
     PlayGamesPlatform.Activate();
     GooglePlayLOGIN();
     DontDestroyOnLoad(this.gameObject);
 }

 void OnGUI()
 {
     GUI.Label(new Rect (Screen.width*0.5f,Screen.height*0.5f,1000,30),"11"+GooglePlayGodinID);
 }

 void GooglePlayLOGIN()
 {
     Social.localUser.Authenticate((bool success) =>
       {
          if(success)
         {
             GooglePlayGodinID=PlayGamesPlatform.Instance.GetUserId();
         }else{

         }
     });
 }

How to get Current User's Id? This code returns NULL Help me pls