MenuSnapToGrid.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using UnityEditor;
using UnityEngine;

[InitializeOnLoad]
static class AddOnMenu {
    [MenuItem("Edit/Snap to Grid %g")]
    static void MenuSnapToGrid() {

        // read-cache preferences found in (EDIT -> SNAP SETTINGS)
        float snapX = EditorPrefs.GetFloat("MoveSnapX");
        float snapY = EditorPrefs.GetFloat("MoveSnapY");
        float snapZ = EditorPrefs.GetFloat("MoveSnapZ");

        // iterate each selected, top-level & modified allowed
        foreach (Transform transform in Selection.GetTransforms(SelectionMode.OnlyUserModifiable | SelectionMode.TopLevel)) {

            // prepare undo configuration
            EditorGUI.BeginChangeCheck();

            // record previous state
            Undo.RecordObject(transform, "Snap to Grid");

            // adjust object
            transform.position = new Vector3(Mathf.Round(transform.position.x / snapX) * snapX, Mathf.Round(transform.position.y / snapY) * snapY, Mathf.Round(transform.position.z / snapZ) * snapZ);

            // commit undo change
            EditorGUI.EndChangeCheck();
        }
    }
}