ChangeBrakePads

The 1996-and-up GS500E model and 2004-2006 GS500F model brakes are the same. The front/rear rotors and the rear pads will work from ANY year model of GS500 (1989 - 2006).

ROTORS FRONT:
Pro-Lite OE Replacement Rotor Front --- MD3006RS
Pro-Lite Contour Plus Rotor Front ---MD3006C
ROTORS REAR:
Pro-Lite OE Replacement Rotor Rear --- MD3002
Pro Lite Contour Plus Rotor Rear --- MD3002C
PADS FRONT:
Kevlar Organic Pads Front --- FA231 \\ Sintered HH Pads Front --- FA231HH
PADS REAR: Kevlar Organic Pads Rear --- FA63
Sintered HH Pads Rear --- FA63HH
From here

The easiest to find are brake pad replacements from EBC, Kevlar Organic.
* Front: FA231
* Rear: FA63

PRICING EBC Kevlar FA231 and FA63
1) Chaparral $52.97
2) MAW $55.85
3) Ron Ayers $57.44
Prices include SH

Some more good reading on GS brakes.

A video on replacing the rear brake pad: http://www.youtube.com/watch?v=YdxkIgnrolI