GUILabel doesn't show up

It would only show it while the button was held down. You should initialize some time when that message is triggered and then show it while the time is less than that target:

 var message1Time : float;
 var message2Time : float;
 
 function OnGUI (){
     if (showGameButtons){
        // Player ends turn by banking points earned in this turn
        if (GUI.Button (Rect(10,40,100,30),"Bank")){
          if (GameObject.Find("player(Clone)"+activePlayer).GetComponent("PlayerAttributes").score==0){
           if (PreScore.guiScore>=1000){
               GameObject.Find("player(Clone)"+activePlayer).GetComponent("PlayerAttributes").score += PreScore.guiScore;
               endTurn();
           }else{
               print("shikaka");
               message1Time = Time.time + 4;
           }
          }
          if (GameObject.Find("player(Clone)"+activePlayer).GetComponent("PlayerAttributes").score>0){
           if (PreScore.guiScore>=350){
               GameObject.Find("player(Clone)"+activePlayer).GetComponent("PlayerAttributes").score += PreScore.guiScore;
               endTurn();
           }else{
               message2Time = Time.time + 4;
           }
          }
        }
        if(message1Time > Time.time)
        {
               GUI.Label(new Rect((Screen.width-500)/2,Screen.height/2-15, 520, 22), "You need atleast 1000 points to open the game!",alertStyle);
        }
        if(message2Time > Time.time)
        {
               GUI.Label(new Rect((Screen.width-500)/2,Screen.height/2-15, 520, 22), "You need atleats 350 point to bank!",alertStyle);
        }
        if (GUI.Button (Rect(130,40,100,30), "Keep")) {
          // to press this button preScore must be bigger than 0;
          if (PreScore.preScore>0){
           PreScore.turnScore += PreScore.preScore;
           PreScore.preScore = 0;
           gameObject.GetComponent("PreScore").reset = true;
           gameObject.GetComponent("Roll").enabled = true;
           print(PreScore.preScore);
           KillDice();
 
          }else{
          // pazinojums, ka jaatzime vismaz viens kaulins
          }
        }
     }
     if (!showGameButtons){
        GUI.Label(new Rect((Screen.width-500)/2,Screen.height/2-15, 520, 22), "You can't score anything with these dice!",alertStyle);
        if (GUI.Button (Rect(((Screen.width - 120) / 2)+10, Screen.height/2+25,100,30),"End Turn")){
          endTurn();
        }
     }
 }