I'm interested in having those gizmos in-game, so the player can scale, move, rotate, etc. Is there a script somewhere for this feature? and if not, how to approach writing such a script? perhaps someone could try?
Sound rather easy - If you want to keep the gizmo model in the topmost layer in-game, simply use the GL library contained in the Unity engine, or simply use GUI Buttons; GUI Buttons are alot more basic.
Would be very simple to do. Simply linking player input to manipulation of transforms.
All you have to do to start fleshing out a feature like this is to maybe add GUI buttons and some sort of object to manipulate in your scene. Make the GUI buttons affect the various transform parameters; rotation, position, or scale of the objects. Or use keyboard input to do it, but either way. Maybe even allow values to be manually typed in (though that's typically not user-friendly).
For something more fully-featured, you would of course want to allow the player to select objects, but that would be as simple as setting the GameObject being manipulated to the last clicked object.
I would recommend reading the manual if you don't know how to write any of this though, because this can all be accomplished with the most basic functions of Unity scripting.