Actually, I just tested it, you don't even need my .gobjs, just two .gfxs. Ok, hopefully you can download the two .gfx files from here:
http://www.mediafire.com/file/a10v3suh9x...xample.zip
Alright, so start out with a blank screen in the editor. Now create any sized window you want, default style will be fine.
Now, in that window create a rectangle with square.gfx. Make the rectangle's width longer than the height. Now, for the rectangle go to Color Add and change the color to something other than black. Go to Color and change the opacity so that it is slightly translucent. These don't really matter, but just for example.
Now, on top of that rectangle use button.gfx and create a button, the size doesn't matter. Now duplicate that button 2 more times. Just make sure they're on top of that big, semitranslucent rectangle.
To show you what I mean, here's a screen.
Now save. Close the editor. Reopen the editor. Reopen the GUI file.
You don't really need to do any of those specific things to run into this problem. It really happens with anything.