GridMap

using UnityEngine;
using System.Collections;

[RequireComponent(typeof(BoxCollider))]
public class GridMap : MonoBehaviour {

    public Vector3 GridPosition;
    public Vector2 GridScale = Vector2.one;
    public bool DependsOnObjectTransform;

    private BoxCollider gridCollider;

    [Range(1,50)]
    public int GridWidth = 1;
    [Range(1,50)]
    public int GridHeight = 1;
    private Vector2 cellSize;
    private Vector3 gridWorldBottomLeft;

    public Cell[,] grid;

    private Vector3 mousePoint;

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

    public void OnValidate () {
        MapGenerate ();
    }

    public void MapGenerate () {
        grid = new Cell[GridWidth, GridHeight];
        cellSize = new Vector2 (GridScale.x / GridWidth, GridScale.y / GridHeight);

        if (DependsOnObjectTransform) {
            GridPosition = transform.position;
            GridScale = Vector2.right * transform.lossyScale.x + Vector2.up * transform.lossyScale.y;
        }

        if (gridCollider == null)
            gridCollider = GetComponent<BoxCollider> ();
        gridCollider.size = Vector3.right * GridScale.x + Vector3.up * GridScale.y;
        gridCollider.center = GridPosition;
        gridWorldBottomLeft = GridPosition - Vector3.right * GridScale.x / 2 - Vector3.up * GridScale.y / 2;
        for (int x = 0; x < GridWidth; x++) {
            for (int y = 0; y < GridHeight; y++) {
                Vector3 worldPoint = gridWorldBottomLeft + Vector3.right * (x * cellSize.x + cellSize.x / 2) + Vector3.up * (y * cellSize.y + cellSize.y / 2);
                grid [x, y] = new Cell (true, worldPoint);
            }
        }
    }

    void Update () {
        RaycastHit hit;
        if (gridCollider.Raycast (Camera.main.ScreenPointToRay (Input.mousePosition), out hit, 40f)) {
            mousePoint = hit.point;
        }
    }

    Cell GetCellFromWorldPoint (Vector3 worldPoint) {
        Vector3 correctedPoint = Vector3.one - worldPoint;
        float percentX = (correctedPoint.x + GridScale.x / 2) / GridScale.x;
        float percentY = (correctedPoint.y + GridScale.y / 2) / GridScale.y;
        Debug.Log (worldPoint + " | " + worldPoint);

        percentX = Mathf.Clamp01 (percentX);
        percentY = Mathf.Clamp01 (percentY);

        int x = Mathf.RoundToInt((GridWidth - 1) * percentX);
        int y = Mathf.RoundToInt((GridHeight - 1) * percentY);

        return grid [x, y];
    }

    void OnDrawGizmos () {
        Gizmos.DrawWireCube (GridPosition, Vector3.right * GridScale.x + Vector3.up * GridScale.y);
        if (grid != null) {
            Cell playerCell = GetCellFromWorldPoint (mousePoint);
            foreach (Cell c in grid) {
                Gizmos.color = Color.white;
                if (c.Accessible)
                    Gizmos.DrawWireCube (c.WorldPosition, Vector3.right * cellSize.x + Vector3.up * cellSize.y);
                if (playerCell == c) {
                    Gizmos.color = Color.red;
                    Gizmos.DrawCube (c.WorldPosition, Vector3.right * cellSize.x + Vector3.up * cellSize.y);
                }
            }
        }
    }
}

public class Cell {
    public bool Accessible;
    public Vector3 WorldPosition;

    public Cell (bool _accessible, Vector3 _worldPosition) {
        Accessible = _accessible;
        WorldPosition = _worldPosition;
    }
}