How to detect a button being released?

If I'm understanding your question correctly, I would do something like this:

 var counter: int = 0; 
 var Timer = 0.0;
 var gameText: GUIText;
 var btnStyle:GUIStyle;
 var backGround:GUIStyle;
 
 //A bool to let us know if the button has been pressed or not
 var down = false;
  
 function Start(){ 
     gameText = gameObject.GetComponent(GUIText); 
 }
 function OnGUI () {
 GUI.Box (Rect (100,100,200,300), "Hold IT", backGround); 
     if (GUI.RepeatButton(new Rect(50,50,100,110),"", btnStyle)) {
     //when the button is pressed we make down equal true
     down = true;
     counter++;
     Timer += Time.deltaTime;
     var hours : int =  Timer / 3600;
     var minutes : int = Timer / 60;
     var seconds : int = Timer % 60;
     var fraction : int = (Timer * 100) % 100;
     gameText.text = String.Format ("{0:00}:{1:00}:{2:00}:{3:00}", hours, minutes, seconds, fraction);  
     }
     else if(down == true && Event.current.type == EventType.MouseUp) { 
            //An else if statement that will only be true if the button is not 
            //pressed but has been pressed in the past and the mouse has been lifted
            //but to check in the OnGUI function if the mouse has lifted we need to use Event.current and 
            //check it against the event type of mouse up which will only be true if the mouse has been pressed down and released
            //Thanks to BoredMormon for the Event.current == EventType.MouseUp part! This code wouldn't work without it!

            //Game over code
            //Remember to set down to false every time a new game is started or else
            //the game over code will run right at the start of the game!
            //Also remember that if you don't set down to false after the game over code every time you click the mouse and release the game over code will run
     }
   }  

Here is some more info about Events:
http://docs.unity3d.com/ScriptReference/Event.html

Hope that helps! Let me know if it didn't answer your question!