Have you looked into HiViz sites? I have one on my SR-22 and it works well. The photos below are the two sites I saw for the GP 100 on their website.
You could use whiteout to paint the existing front site if you want a super easy, low cost, not-long-lasting white front site. It does help make the site more visible but it does not last (my dad used to do this when his eyes started preventing him from seeing the site). Some guys I shoot with have used Testors model paint but say it also wears off with regular cleaning of your gun. I believe there’s actual site paint in lots of color choices, too, but I’ve never looked into that.