How can I access a GameObject variable in mono script from editor script?

In the mono script :

using UnityEngine;

public class TileTest : MonoBehaviour
{
    public GameObject prefabPathOnTerrain;

Then in the editor script at the top :

private SerializedProperty prefabPathOnTerrain;

Then inside OnEnable()

private void OnEnable()
    {
        prefabPathOnTerrain = serializedObject.FindProperty("prefabPathOnTerrain");
    }

And then inside a method I want to use it as a GameObject : And instead using CreatePrimitive to use Instantiate : But doing :

GameObject go = Instantiate(prefabPathOnTerrain);

Will not work.

public void TerrainObjects(GameObject gameobject)
    {
        Terrain terrain = Terrain.activeTerrain;
        var cube = GameObject.Find(string.Format("Terrain_{0}_{1}", gameobject.transform.localPosition.x, gameobject.transform.localPosition.y));
        if (cube != null)
            return;

        if (cube == null)
        {
              GameObject go = Instantiate(prefabPathOnTerrain);              

            //cube = GameObject.CreatePrimitive(PrimitiveType.Cube);
        }

        cube.transform.localPosition = new Vector3(terrain.transform.localPosition.x + gameobject.transform.localPosition.x, 0, terrain.transform.localPosition.z + gameobject.transform.localPosition.y);
        cube.transform.localScale = new Vector3(gameobject.transform.localScale.x, gameobject.transform.localScale.y, 1);
        cube.transform.parent = GameObject.Find("Terrain Map").transform;

        cube.name = string.Format("Terrain_{0}_{1}", gameobject.transform.localPosition.x, gameobject.transform.localPosition.y);
        cube.tag = "Terrain Map";
    }