i have thoroughly enjoyed the GP100 4" I traded with CSW for. I use 125-grain Speer Gold Dot in .357 Magnum and 135-grain Speer Gold Dot in.38 Special +P. Both are pleasant through the GP100. I've also tried Sellior and Bellot lead round nose .357 Magnum and Winchester Ranger 130-grain .38 Special +P. All were great.
My favorite part about owning the GP100 so far was when I took my 5'2", 125 pound girlfriend to shoot it. I assumed she'd do better with the .38 Specials. She ended up not only preferring the .357 loads, but shooting them significantly better. I can't explain it. I definitely did better with the .38s, so I guess we know who wears the pants.
The .357 loads feel stout but well controlled. The 38s recoil very light in that beefy frame.
It's very hard to go wrong with choosing a load for the GP. You can't hurt it.