Instantiated objects can't convert to GameObjects

Hi all, so I am trying to clear all instantiated objects from scene. So, I am putting all of the instantiated objects in a list, and destroy them in a loop. My code currently looks like this:

     private List<GameObject> arlist = new List<GameObject>();
     private float f;
     public void PerlinGen()
     {
         for (int x = (int)this.transform.position.x-500; x <= (int)this.transform.position.x+500; x+=10)
         {
             for (int z = (int)this.transform.position.z-500; z <= (int)this.transform.position.z+500; z+=10)
             {
                 f = Mathf.PerlinNoise((x+500f)/1000f,(z+500f)/1000f);
                 GameObject k = (GameObject)Instantiate(prefab, new Vector3(x, Convert.ToSingle(Math.Round(f,2))*1000, z), Quaternion.identity);
                 arlist.Add(k);
             }
         }
     }
     private float time = 30f;
     public float interpolationPeriod = 2f;
     public void DestroyStuff()
     {
         foreach (GameObject cube in arlist)
         {
             Destroy(cube);
         }
         arlist.Clear();
     }
     void Update() {
         time += Time.deltaTime;
     
         if (time >= interpolationPeriod) {
             time = 0.0f;
             Debug.Log("ready");
             DestroyStuff();
             PerlinGen();
         }
     }

Well, Unity says you can't convert from Transform to GameObject, specifically in this line:

 GameObject k = (GameObject)Instantiate(prefab, new Vector3(x, Convert.ToSingle(Math.Round(f,2))*1000, z), Quaternion.identity);

I tried to use as, but Unity raises

Assets\worldGen.cs(21,32): error CS0039: Cannot convert type 'UnityEngine.Transform' to 'UnityEngine.GameObject' via a reference conversion, boxing conversion, unboxing conversion, wrapping conversion, or null type conversion

So,basically I tried fixes on the Internet but to no avail. Can you help me please?