A couple of bugs:
(put the .cap in the same folder as the original demo .cap)
It seems that when you add a sprite to the layout, it's not that the sprite is rendering behind the GUI, but that the sprite isn't rendering at all. I resized the GUI so that you should be able to see the sprite, but it's not there.
Also, run the .cap and hold down the right arrow to move the (invisible) sprite. When the screen scrolls to follow the sprite, the GUI disappears. This doesn't happen when the GUI layer is set to 0% scroll rate.
And there's some minor visual bugs in the layout that you're probably already aware of. When you click anywhere in the object panel, in the layout, on a layer, or on a scrollbar, the animation on the GUI object jumps ahead. This also happens when hitting any key on the keyboard. And scrolling the layout causes the background image of the GUI to squish and stretch, both horizontally and vertically.