概要

using System;
using System.Reflection;
using UnityEditor;

internal static class Example
{
    [MenuItem( "Tools/Run" )]
    private static void Run()
    {
        const BindingFlags bindingAttrs = BindingFlags.Instance | BindingFlags.NonPublic;

        var gameViewType            = Type.GetType( "UnityEditor.GameView,UnityEditor" );
        var showToolbarPropertyInfo = gameViewType!.GetProperty( "showToolbar", bindingAttrs );
        var editorWindow            = EditorWindow.GetWindow( gameViewType );
        var showToolbar             = ( bool )showToolbarPropertyInfo!.GetValue( editorWindow );

        showToolbarPropertyInfo.SetValue( editorWindow, !showToolbar );
    }
}