You can make sure that gui-elements are aligned when their parents are resized with the setAlignment function. So you can create the gui in one resolution and for example set a gui-element with that size as parent which does draw nothing (for example a IGUIWindow without titlebar and removed buttons or a IGUIStaticText without border and text). Now when you resize that parent it's childs will all align correctly - for example to the center.
You can not center the application window itself, at least not with Irrlicht functions. You can use platform API functions for that.