There's a few GP100 variants you can only find used. The most amazing to me is the 6" barrel half-lug...basically a "light barrel" variant no longer in production. Has the "nose flick" feel of a 4" barrel, but with the accuracy and velocity of a 6".
("Accuracy" is mainly due to the longer sight radius - in other words, sights farther apart are easier to do fine alignment with.)
I do suggest reading up on the "checkout thread" - new or used. Ruger produces lemons sometimes.
An odd thing about Rugers in general and the GP100 in particular is that they "shoot fast". Take two 4" barrel 357s, one a classic S&W like the 19, the other a GP100, the GP100 could have as much as an extra 100fps of heat on the rounds. That's like a 4" barrel Ruger shooting as fast as a 6" barrel S&W. Sweet. S&W's newest variants have almost caught up in that race...
One argument for new is that Ruger's quality control of late has been the best seen since the early '70s. My 2005-vintage Ruger NewVaq 357 bought new was perfect as were the other three I ran the checkout on, and complaints on any other Ruger wheelgun are very scarce.
But armed with the checkout you can scope out problem children, or those still "in spec" but not desirable (too much barrel/cyl gap, sloppy action, funky trigger, etc.). Problems can happen with any production gun - the only revolvers I'd buy sight unseen would be Freedom Arms or Korth...