I modified a mount I had bought for a Super Blackhawk. Had a 2x Leupold on it.
Mounts at the rear sight and clamps to the barrel, had in on my gun for years kept Zeroed even with hot .357 Hand Loads.
It's a B-square mount the clamps I made out of stainless the originals were aluminum.
Had to sell the GP100 years ago when I needed money and had no job.