Rear Bumper, Side Skirts and Alloys (5x112) are bolt ons. The rear GTI spoiler is an integral part of the tailgate so you will need to buy the complete set unless you get an aftermarket version and glue it on.
The front end depends on what bumper you have currently have, the earlier GT bumper is the same as the standard Golf so you will need a GTI front carrier to do the job properly. But some people take a shortcut by trimming the GTI bumper to fit over the standard carrier which does fit eventually but you lose access to the front tow. The later GT/GT Sport bumper shares the same design as the GTI which makes the conversion much easier, all you need is to changeover the grilles to complete it.