Can't go wrong with the 4" GP100. Buy the Wilson Combat spring kit for $10 and Hi Viz brand front sight kit to finish it off. The front sight comes out using a plunger on the front of the barrel. It's spring loaded. Pull the old sight out and slide the new one in. Done. They give you red, hi viz red, white and hi viz green light pipes with the kit. I hit targets at 100 yards easily with mine.
My buddy and fellow GP owner says to get the Wolf kit. Is the Wilson kit better? I saw a video on youtube on how easily the front sight can be replaced, that's AMAZING!