Need help calling a script to another keep getting errors

I keep getting error CS0119, error CS1502 and error CS1503 from trying to call one script to another script function. I'm trying to access my game over script whenever the player dies it's called, but some reason I just get those errors here is the scripts for reference. Error happens in the Void Start function.

 public Gameover GameOverScript;
 
 void Start ()
     {
 GameOverScript = GameObject.FindGameObjectWithTag ("GameOver").GetComponent (Gameover);
     }
 
 
         void OnGUI()
     {
                 if (playerLives != 0) {
             for (int i=1; i<=playerLives; i++) {
                 GUI.DrawTexture (new Rect (i * 36, 0, 750, 48), shipTexture, ScaleMode.ScaleToFit, true);
             }
                         }
                 }
 

Here is my gameover script

    public class Gameover : MonoBehaviour {

 void OnGUI()
 {
     const int buttonWidth = 120;
     const int buttonHeight = 60;
     
     if (
         GUI.Button(
         // Center in X, 1/3 of the height in Y
         new Rect(
         Screen.width / 2 - (buttonWidth / 2),
         (1 * Screen.height / 3) - (buttonHeight / 2),
         buttonWidth,
         buttonHeight
         ),
         "Retry!"
         )
         )
     {
         // Reload the level
         Application.LoadLevel("Stage1");
     }
     
     if (
         GUI.Button(
         // Center in X, 2/3 of the height in Y
         new Rect(
         Screen.width / 2 - (buttonWidth / 2),
         (2 * Screen.height / 3) - (buttonHeight / 2),
         buttonWidth,
         buttonHeight
         ),
         "Back to menu"
         )
         )
     {
         // Reload the level
         Application.LoadLevel("Menu");
     }
 }

}

That would be great if anybody can help me