See what or if XS Sight systems has anything for the GP100....in fact, here you go....http://xssights.com/index.php?nID=si...ts&sID=handgun
... Or find and put a fiber optic on it and though not the same gun, I was window shopping today on gunbroker.com and saw an SP101 with a fiber optic front sight? So, maybe someone makes one for the GP100? But the link to XS should be what you need and want? Just click Ruger, then your model and ta-da.
But here's the link to the SP101 so you can see for yourself how it looks, or would look if someone, like XS systems, makes a fiber optic for the GP100 and you like how it looks and works, maybe you can find one? Ruger SP101 357 5 Shot with FIBER OPTIC sight NIB! : Revolvers at GunBroker.com
Also here's a link to an auction for a front sight for a GP100 that has a red insert in it instead of being all black. Ruger - RFS - Gp-100 Front Sight W/Red Insert : Gun Sights at GunBroker.com
Also, in the mean time, you might or could try some fluorescent paint. Like fluorescent orange or green or something. Or even some of that glow paint specifically for sights? Some people have even been known to even paint their all black front sights with a bright fingernail polish. And it holds up pretty well as it's an enamel.