Calling a function from other script in Unity

I am trying to call a function from a script that is attached to a game object using an other script which is attached to a second game object.

The two game objects are called: Player, Game Manager.

Player has a script called Gold and Game Manager has a script called GameManager

Trying to call SetGold() function in Gold.cs from GameManager.cs, but I am getting the error: NullReferenceException: Object reference not set to an istance of an object

My current code: Gold.cs:

using UnityEngine;
using System.Collections;

public class Gold : MonoBehaviour {
    GameManager gm;

    public UnityEngine.UI.Text goldDisplay;

    private float _gold;

    void Start ()
    {
        gm = GameObject.Find("Game Manager").GetComponent<GameManager>();
        gm.Load();
    }

    void Update ()
    {
        UpdateGoldDisplay();
    }

    public void SetGold(float x)
    {
        _gold = x;
    }

    public float GetGold()
    {
        return _gold;
    }

    public void UpdateGoldDisplay()
    {
        SetGoldDisplay(GetGold().ToString());
    }
    public void SetGoldDisplay(string x)
    {
        goldDisplay.text = x;
    }
}

my other script (GameManager.cs)

using UnityEngine;
using System.Collections;

public class GameManager : MonoBehaviour {

    GameObject GO_Player;
    Gold gold;

    void Start ()
    {
        GO_Player = GameObject.Find ("Player");
        gold = GO_Player.GetComponent<Gold>();
    }

    void OnApplicationQuiot()
    {
        Save();
    }
    public void Load()
    {
        gold.SetGold(PlayerPrefs.GetFloat("gold", 50));
    }
    private void Save()
    {
        PlayerPrefs.SetFloat("gold", 1);
    }

    public void DeleteSaves()
    {
        PlayerPrefs.SetFloat("gold", 0);
    }
}

Thanks in Advance.