The GP100 pinned in sight probably will work but you will need to modify it to get it to fit.
Remove your old sight and place it against the new sight (with the flat parts of the bottoms aligned, not the tops) and scribe where you will need to remove material at the bottom to have it fit (if any). Place it the slot and use something to hold it in place and drill the hole from both sides to insure it is correct. Then put the pin back in and see how it shoots.
Also, the weight of the bullet makes a big difference in the point of impact. The Six series revolvers were pretty much designed for 158 gr bullets, so that is how Ruger set up the sights.