The time is calculated from the difference of Time.time and startTime. To reset your timer, you need to make this difference be zero, i.e.

void ResetTimer()
{
    startTime = Time.time;
}

void OnGUI()
{
    // your code here
    //...

    if(GUI.Button(new Rect(50, 10, 200, 40), "Reset")){
        Debug.Log("Reset Time");
        ResetTimer();
    }

This will work for you 100%. D

private gameTime=120;

void Start () 
{
    CountDownSeconds=gameTime;

    startTime = Time.time;
}

void Update () 
{

    Timeleft = Time.time-startTime;
    restSeconds = CountDownSeconds-(Timeleft);
    roundedRestSeconds = Mathf.CeilToInt(restSeconds);
    displaySeconds = roundedRestSeconds % 60;
    displayMinutes = (roundedRestSeconds / 60)%60;
    timetext = (displayMinutes.ToString() + ":");
    if (displaySeconds > 9)
    {
       timetext = timetext + displaySeconds.ToString();
    }
    else 
    {
       timetext = timetext + "0" + displaySeconds.ToString();
    } 
    GUI.Label(new Rect(10, 10, 200, 40), timetext);

   if(GUI.Button(new Rect(50, 10, 200, 40), "Reset")){
     Debug.Log("Reset Time");
ResetTimer();
   }
    if(roundedRestSeconds==0)
    {
       Application.LoadLevel("LevelLost");
    }
}
public void ResetTimer()
{
    startTime=Time.time;
    CountDownSeconds = gameTime;
}

Let me know if its working fine.

Handle the timer vars in an Update function because OnGUI can be called several times per frame. You can set startTime = 0 if the reset button is pushed.