do our bikes have sintered or non-sintered pads?
whats a good brand??

I'd go with Galfer greens.

Is there a specific model of galfer greens to buy for the GS500 1996?

Thanks

are the brakes for a 2002/2001 and up any different from the other models?

Sintered pads will give you better 'bite' but some say they lack 'feel'.  In other words, you'll stop quicker but have to be more careful about locking up.  

I've often seen it recommended to have sintered pads on the front and non-sintered e.g. kevlar, organic etc on the back.  

From personal experience, I recommend EBC HH sintered pads.  They transformed the front brake on the GS when I first swapped out the OEM pads.