Mike Dailly, the creator of Lemmings and GTA, has made the design documents for Grand Theft Auto 2 available on his website. As well as docs for GTA2, the website also features design information and history for Lemmings and the original GTA.
You can find the GTA docs by going to http://lemmingsgame.com -> Pictures/Projects -> GTA (Content warning. Does contain some homophobic language)