For many years Ruger catalogued .38 Special only versions of the GP100. Most, if not all of them were fixed-sight, compact grip varients, as Ruger perceived their primary appeal to LEO/security orgaziations on a budget and wanting to provide a quality revolver without the additional training time/expense needed to train and familiarize their individuals on the increased power, recoil, and muzzle flip inherent to the .357 magnum cartridge. The ones that I've seen over the years have tended to be slower sellers, as most shooters usually prefer at least to have the option of loading .357 magnum, and subsequently are a bit lower priced.
Personally, I agree with GP100man-I almost always use .38 Special +P ammunition in both my GP100 and Security Six in both competition and carry, for reasons of increased controllability and decreased recovery times between shots. But I do keep some .357 magnum ammunition on hand, and I like having the option to use it...
Best, Jon