The cities in the GTA series don't replace the real-life cities on which they are based, but exist alongside them. We know this from GTA: Vice City, which mentions the real life Times Square as well as fictional Liberty City. This doesn't mean that RDR and GTA take place in the same universe, but isn't evidence that they don't either.
That said, even if the two series take place in the same universe, the MacFarlane ranch is located in the state of New Austin (based on Texas \ New Mexico \ Arizona) and not in Los Santos' state of San Andreas (based on California \ Nevada).
Last edited by SCAB; 07-29-2013 at 04:27 AM..