Any holster for a S&W K or L frame will also fit the GP100. I assume you're looking for a vertical holster? Take a look at the Bianchi X15 and the Triple K Challenger vertical shoulder holsters. They're kind of old school, but they both work well and they're priced very reasonably.
“If ye love wealth better than liberty, the tranquility of servitude better than the animating contest of freedom, go home from us in peace. We ask not your counsels or arms. Crouch down and lick the hands which feed you. May your chains set lightly upon you, and may posterity forget that ye were our countrymen.” ― Samuel Adams