GridLayoutGroupCells.cs

using UnityEngine;
using UnityEngine.UI;

/// <summary>
/// Use a number of cells and columns instead of cell width and height
/// </summary>
[RequireComponent(typeof(GridLayoutGroup))]
[ExecuteInEditMode]
public class GridLayoutGroupCells : MonoBehaviour
{
    GridLayoutGroup gridLayoutGroup;
    RectTransform rectTransform;
    [SerializeField] int columns = 0;
    [SerializeField] int rows = 0;

    private void Awake()
    {
        gridLayoutGroup = GetComponent<GridLayoutGroup>();
        rectTransform = GetComponent<RectTransform>();
    }

    private void Update()
    {
        CalculateCellSize();
    }

    void CalculateCellSize()
    {
        float cellWidth = rectTransform.rect.width;
        if (columns > 0)
        {
            cellWidth -= gridLayoutGroup.padding.left;
            cellWidth -= gridLayoutGroup.padding.right;
            cellWidth -= gridLayoutGroup.spacing.x * (columns - 1);
            cellWidth -= float.Epsilon * 2;
            cellWidth /= columns;
        }

        float cellHeight = rectTransform.rect.height;
        if (rows > 0)
        {
            cellHeight -= gridLayoutGroup.padding.top;
            cellHeight -= gridLayoutGroup.padding.bottom;
            cellWidth -= gridLayoutGroup.spacing.y * (rows - 1);
            cellHeight -= float.Epsilon * 2;
            cellHeight /= rows;
        }

        gridLayoutGroup.cellSize = new Vector2(
            cellWidth,
            cellHeight
        );
    }
}