The beta branch on Steam is updated with a new build. This uses the refactored Window class, on Windows and Linux. The GUI and Widget class are also added, although they are highly experimental and still in development. Both the engine and editor are using the refactored Window class, so please report any erroneous behavior your detect.
Leadwerks is now using GDI+ for some GUI drawing commands, on Windows. You need to update your existing project by modifying the "Linker \ Input \ Additional Dependencies" property in Visual Studio and adding the "msimg32.lib" library.
If you want to play around with the GUI system, check out GUI.h and Widget.h, in the Pro edition.