I second the recommendation for the HiViz fiber optic front sight. The factory front ramp sight on the GP100 is a bad joke as it is nigh invisible against a dark background/target.
The HiViz sight is available directly from Ruger for $35 (plus postage), or you can get it from Amazon.com (
here) for a little over $28 with free shipping.
The hammer/trigger spring kit mentioned in a previous post also does wonders for improving the trigger pull.