HowTo pass variable to OnGUI

I have a huge problem which i can solve since 4 days now. I have to say that im very new to unity and C+

My Problem is that i can not access a variable set on a public function on the same script. They are like 2 different values for the same variable.

I have 2 scripts. HealthBarMonster calls the ShowScore script and passes the score value when a GameObject gets destroyed. But i can not access the variable set in CalculateScore with my onGUI function. What i am doing wrong, thank you for any help i get

HealthBarMonster

 using UnityEngine;
 using System.Collections;
 
 public class HealthBarMonster : MonoBehaviour {
 
     public float MaxHhealth = 100f;
     public float CurrentHealth = 0f;
     public int Score = 10;
     public GameObject healthBar;
     public Tower Tower;
     public ShowScore ShowScore;
 
     // Use this for initialization
     void Start () {
         CurrentHealth = MaxHhealth;
 
     }
     
     public void decreasehealth(float dmg){
         CurrentHealth -= dmg;
         float calc_health = CurrentHealth / MaxHhealth;
         SetHealthBar (calc_health);
     }
 
     public void SetHealthBar(float health){
         healthBar.transform.localScale = new Vector3 (Mathf.Clamp (health, 0f, 1f), healthBar.transform.localScale.y, healthBar.transform.localScale.z);
         if (CurrentHealth <= 0) {
             Tower.DeathReport ();
             ShowScore.CalculateScore (Score);
             Destroy (gameObject);
         }
     }
 }

ShowScore

 using UnityEngine;
 using System.Collections;
 
 public class ShowScore : MonoBehaviour {
 
     private int ScoreCount = 0;
     private int ScoreCountGUI = 0;
 
     public void CalculateScore (int score){
         if (score != 0) {
             ScoreCount += score;
         } else {
             ScoreCountGUI = ScoreCount;
         }
     }
         
     public void OnGUI()
     {
         CalculateScore (0);
         int w = Screen.width, h = Screen.height;
         GUIStyle style = new GUIStyle();
         Rect rect = new Rect(0, 10, w, h * 2 / 100);
         style.alignment = TextAnchor.UpperLeft;
         style.fontSize = h * 2 / 100;
         style.normal.textColor = new Color (0.0f, 0.0f, 0.5f, 1.0f);
         GUI.Label(rect, "Score: "+ScoreCountGUI, style);
     }
 }
 

I know the ScoreCount variable in the if condition "!= 0" goes up right. but in the else condition it is not the same value