Any cat back system for the Toyota Aristo Twin Turbo (JZS147) will fit the GS, although they are pricey, unless you can find one second hand. They share the same factory exhaust sytem apart from downpipes etc. The HKS Silent High Power cat back, with twin exit is £500 + vat, and is only mild steel up to rear boxes. A custom setup will be allot cheaper and you can choose how you want it to sound etc.
Finding rare JDM Aristo exhausts isnt an easy task here in the UK, there are some lovely ones though!