You have to set all the 'units' properties of all the controls to 'normalized'. To do this, type control-a to select everything in the GUI, then set the units property to 'normalized'. This may/will not set the units of controls inside a groupbox panel so you may have to set those separately.
Then, in the OpeningFcn() of your GUI, call the attached MaximizeFigureWindow (if you're using Microsoft Windows) to maximize the window. This truly maximizes it, unlike setting the position property of the GUI. Setting the Position property always seems to leave the window slightly misaligned anyway - bottom left is never at 0,0 like it should be.