I removed the side rails, you need to pop the top of the rack look underneath and you'll see two plastic prongs. These hold the two halfs of the top plate together, once split you can get access to the bolts that hold the side rails on.
The bottom half of my top plate has loads of holes in it, so it was easy to line up on the original GS rack to fit mounting bolts. The rear slots in the GS rack are ideally positioned for mounting points.
I used the mounting points for the GS grab rail to secure the rear of the plate (grab rail was discarded) Beware when you unbolt the grab rail the tailight falls off You can see the four mounting bolts in the picture