How do I make a GUI always be at the bottom of a screen regarding computer screen size?

I want to be able to make those black borders at the top and bottom of a screen like in a movie. I got the black GUI bars and all I need to do is position them. The GUI that goes on top is                     "Position = {0, 0},{0, 0}". It has been tested that it will be at the top of any screen regarding the size.
Now when I tried positioning the bottom GUI to the scale of the bottom of MY screen, it worked, but on my brothers computers, which has a different screen, the GUI is not at the bottom of HIS screen. 
How do I make a GUI always be at the bottom of a screen regarding computer screen size? I have seen it happen in Daniel K's place called "Furniture Showcase [WIP]"