Before the GP100, Ruger made a different, smaller series of DA sixshooter 357s - the "Service/Security Six series".
The "Service Six" was fixed-sight, "Security Six" is adjustable...other than that, basically the same gun. Most people have no idea how cool these were and they tend to go for much less money than they're worth. In reality, their size is just a hair bigger than the S&W K-Frames such as the 19 and 66, but the Rugers were stronger. Not as tough as the later GP100 but not too far off it.
The S&W K-frames had a weak spot, a place at the bottom edge of the barrel where it screws into the frame that had a "flat point" which could crack. Despite being similar in size, the Ruger Service/Security guns had no such flaw.
S&W then came out with the L-Frame, a larger gun that also solved the barrel weakness issue and was designed for heavy use of full magnum loads (per S&W). The GP100 was Ruger's response, a gun that clearly looked to be as big, beefy and tough as the L-frame. Ruger took the opportunity to design the GP100 to be cheaper to manufacture than the hand-fitting needed for the Security/Service series, and discontinued the older guns. Most people assumed it was because the older series was "inferior".
On the contrary: those old Rugers were nearly as tough as an S&W L-Frame while being similar in size to the K. How is that not cool?