i like a belt holster similar to a Yaqui Slide holster. The Yaqui Slide holds a gun high and tight as your belt can stand. It should work good for your GP100. The cylinder and most of the weight would be in the holster at belt level.
Galco Combat Master, works very well,rides high carries the gun securely and tight against the body. Make sure you have a good stiff belt to carry a gun that heavy. It will also make a decent holster feel like a great holster. I use a Mernickle belt made for concealed carry but Galco makes a nice dress belt.
Many belt makers out there but it's a necessity if you're carrying a K frame or heavier.