Welcome to the forums! Tips: Take an old toothbrush to the range, to knock any fouling out from under your extractor when you reload. Keep this area dry, to keep fouling from sticking. These steps along may solve the problem.
If you are anywhere near SE or Central Texas, or even the DFW area, I will look at if for you, if we can coordinate a time to meet during my upcoming vacation time. I am no gunsmith, but I am familiar with the GP100 and some of its trouble spots. I will not work on your gun, but might be able to pinpoint a problem area for you. If you are not close to the area, perhaps another guy like me can take a look at your GP.
I used to keep an old toothbrush in my back pocket on qual day, and during training classes, when I still used sixguns as duty handguns. I have had both S&W and Ruger sixguns get junk under the extractor star, stuck either to the underside of the star itself, or to the cylinder where the star comes to rest. If the weapon's tolerances are tight enough, just a tiny bit of fouling, from dumping one load of empties, can gum things up.
I should note that it is mostly the dirtier-burning practice ammo that does this. Premium duty/carry ammo usually burns cleaner.
Interestingly, my first GP100, purchased in '91 or '92, has grunge grooves on the cylinder, under the extractor star, to minimize the chance of dirty ammo causing problems. I recently purchased another older GP100 with this same feature. I don't know when Ruger dropped this feature, but a GP100 made in 2002, that I bought in 2004, does not have the grunge grooves.
At least one gunsmith would add such grunge grooves as a custom feature. I think it was the late Andy Cannon, but cannot recall with certainty.
One more thing: Run a fingertip under your extractor star, to feel for burrs or irregularities.
Enjoy your GP!
