Recently got this used GP 100. Did a bit of research on a coupla forums and figgered out I could polish it with Mother's Mag Wheel Polish. I was amazed at how quickly the finish became really nice!
I used my Dremel tool in a few places but ended up goin' wild with it. Original idea was to polish it up a bit, it's become almost a mirror finish.
I also spent some time sanding and refinishing the wood portion of the grips.