Editor extension to group selected objects pressing Ctrl+G.

GroupGameObjects.cs

using System;
using UnityEditor;
using UnityEngine;

public static class GroupGameObjects
{
    [MenuItem("Edit/Group Items %g")]
    public static void GroupSelected()
    {
        if (Selection.gameObjects.Length > 1)
        {
            // Calculate selection center
            Vector3 center = Vector3.zero;
            foreach (var go in Selection.gameObjects)
            {
                center += go.transform.position;
            }
            center /= Selection.gameObjects.Length;

            // Spawn new GameObject
            GameObject group = new GameObject("Item Group");
            group.transform.position = center;
            
            using (new UndoGroup($"Group {Selection.gameObjects.Length} objects"))
            {
                Undo.RegisterCreatedObjectUndo(group, "");
                
                foreach (var go in Selection.gameObjects)
                {
                    // Set parent
                    Undo.SetTransformParent(go.transform, group.transform, "");
                }
                
                EditorGUIUtility.PingObject(group);
                Selection.activeGameObject = group;
            }
        }
    }
}

public class UndoGroup : IDisposable
{
    private readonly int group;
    
    public UndoGroup(string groupName)
    {
        Undo.SetCurrentGroupName(groupName);
        group = Undo.GetCurrentGroup();
    }

    public void Dispose()
    {
        Undo.CollapseUndoOperations(group);
    }
}