How to make a GridLayout Fullscreen?

Hi guys I’ve explained the problem in this video, you can jump to 1:45 see what is happening:

Basically I want to have the Grid Layout elements to fill the entire screen, I am using a content Fitter but to no use, it is always missaligned someway.

This is the code I am using:

public int numberOfImages;
	public GameObject defaultImage;
	public GameObject holder;
	public GridLayoutGroup gridLayout;


	// Use this for initialization
	void Start () 
	{
		Setup();
	}

	void Setup()
	{
		gridLayout.cellSize = new Vector2(Screen.width, Screen.height);

		for (int i = 0; i < numberOfImages; i++) 
		{
			GameObject g = Instantiate(defaultImage) as GameObject;
			g.SetActive(true);
			g.transform.parent = holder.transform;

		}
	}

I should be using the Reference Resolution to scale things instead of the real screen resolution

	public CanvasScaler mainCanvas;
	public GridLayoutGroup gridLayout;


	void Start()
	{
		gridLayout.cellSize = new Vector2(mainCanvas.referenceResolution.x, mainCanvas.referenceResolution.y);

       [...]
    }