I have that rail going spare if you in the UK. You can have it dirt cheap, its only laying around.
Firstly you need to see which mounts you have under the rear seat, does it like like this:
3x Mounting holes (older gen up to 2010)
2x Mounting holes (2010 onwards)
or like this, with none (my GT):
Although you can mount it using two, those were for the "batwing" grab rails.
I'm pretty sure the GTR pillion seat is recessed slightly to allow for the grab rail under it but I never had a GT pillion seat to 100% confirm, maybe you can albertinhouston?