Loader

loader.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Loader : MonoBehaviour
{
  void Awake()
  {
    GameObject gameManager = GameObject.Find("GameManager");

    if (!gameManager)
    {
      Debug.LogWarning("You need to add Prefab GameManager");
      return;
    }

    //Check if a GameManager has already been assigned to static variable GameManager.instance or if it's still null
    if (GameManager.instance == null)
    {
      //Instantiate gameManager prefab
      Instantiate(gameManager);
    }
  }
}