You will need to use the predefined width and height of the GUI with the internal variables A_ScreenWidth and A_ScreenHeight and make Param2 Gui show how the expression works. For example, if the width of the GUI is 900 and the height is 550:
Gui, Show, % "x" A_ScreenWidth - 900 " y" A_ScreenHeight - 550 " w" 900 " h" 550, The GUI
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