Stan may have a point about Bill Ruger saying the Six series didn't make money for him, but see the following quote from GunWeek.com (October 2004):

"...[T]he introduction of S&W's L-frame guns must have surely brought market pressure to bear on Ruger. To compete, they needed a larger DA revolver suitable for police, military, and defense use. Bill Ruger wasn't going to fool around; he set out to design the new gun around the very hot ammunition it would have to use. His goal was to design the most durable DA .357 Magnum ever. The result was the GP-100, introduced in 1986."

So whether I was right or wrong -- I don't want Stan to think I just made it up. The article clearly ties the design of the GP100 to the introduction of the L-frame revolvers.