GTA games are, as far as I know, mostly built by Rockstar North studio (formerly DMA Design), which is located in Scotland. Obbe Vermeij, former Rockstar North employee, has written some interesting blog posts about the development of GTA from 3 through San Andreas, which can be found archived here: https://gist.github.com/WillPresley/d57d48b3d5f7f7b4bde4a2a3... (he has taken the blog down). It seems that, at least in that era, Rockstar New York was mostly handling the story and voice acting.