GridMesh.cs

using UnityEngine;
using System.Collections.Generic;

[RequireComponent(typeof(MeshRenderer))]
[RequireComponent(typeof(MeshFilter))]
public class GridMesh : MonoBehaviour
{
    public int GridSize;

    void Awake()
    {
        MeshFilter filter = gameObject.GetComponent<MeshFilter>();        
        var mesh = new Mesh();
        var verticies = new List<Vector3>();

        var indicies = new List<int>();
        for (int i = 0; i < GridSize; i++)
        {
            verticies.Add(new Vector3(i, 0, 0));
            verticies.Add(new Vector3(i, 0, GridSize));

            indicies.Add(4 * i + 0);
            indicies.Add(4 * i + 1);

            verticies.Add(new Vector3(0, 0, i));
            verticies.Add(new Vector3(GridSize, 0, i));

            indicies.Add(4 * i + 2);
            indicies.Add(4 * i + 3);
        }

        mesh.vertices = verticies.ToArray(); 
        mesh.SetIndices(indicies.ToArray(), MeshTopology.Lines, 0);
        filter.mesh = mesh;

        MeshRenderer meshRenderer = gameObject.GetComponent<MeshRenderer>();
        meshRenderer.material = new Material(Shader.Find("Sprites/Default"));
        meshRenderer.material.color = Color.white;
    }
}

Thanks for sharing.
I tweaked the code a bit to fit my needs.

1. Removed the bending.
2. Made the grid on the XZ Plane.
3. Added a Vector2Int for the gridSize in case the grid doesn't have the same length and width.
4. Added float for the gridSpacing, simply the size of each cell in the grid.

public class GridMesh : MonoBehaviour {
        public MeshFilter filter;
        public Vector2Int gridSize;
        public float gridSpacing;

        Mesh mesh;
        List<Vector3> verticies;
        List<int> indices;

        private void Start() {
            mesh = new Mesh();
            MeshRenderer meshRenderer = filter.GetComponent<MeshRenderer>();
            meshRenderer.material = new Material(Shader.Find("Sprites/Default"));
            meshRenderer.material.color = Color.black;

            Rebuild();
        }

        private void Update() {
            Rebuild();
        }

        private void Rebuild() {
            verticies = new List<Vector3>();
            indices = new List<int>();

            float xMin = gridSpacing * gridSize.x / 2f;
            float zMin = gridSpacing * gridSize.y / 2f;

            for (int i = 0; i <= gridSize.x; i++) {
                for (int j = 0; j <= gridSize.y; j++) {
                    float x1 = i * gridSpacing - xMin;
                    float x2 = (i + 1) * gridSpacing - xMin;
                    float z1 = j * gridSpacing - zMin;
                    float z2 = (j + 1) * gridSpacing - zMin;

                    if (i != gridSize.x) {
                        verticies.Add(new Vector3(x1, 0, z1));
                        verticies.Add(new Vector3(x2, 0, z1));
                    }

                    if (j != gridSize.y) {
                        verticies.Add(new Vector3(x1, 0, z1));
                        verticies.Add(new Vector3(x1, 0, z2));
                    }
                }
            }

            int indicesCount = verticies.Count;
            for (int i = 0; i < indicesCount; i++) {
                indices.Add(i);
            }

            mesh.vertices = verticies.ToArray();
            mesh.SetIndices(indices.ToArray(), MeshTopology.Lines, 0);
            filter.mesh = mesh;
        }
}