We've recently posted a piece about that fan-made map that placed all five major cities of the GTA franchise on a single large combined continent. At the beginning of that post, we've mentioned the three continuities that can be distinguished in the franchise, and how that fan map throws them out the window.

This post will give you a quick rundown of the three separate universes in the GTA franchise, and will answer a few questions such as why Los Santos looks completely different in GTA V compared to the city of the same name as it appeared in San Andreas, which in turn is completely different from what appeared in GTA...
First of all, we have three universes. These is the 2D universe, the 3D universe and the HD universe.
The 2D universe encompasses GTA, London 1969, London 1961, and GTA II. GTA II incorporated some 3D elements, but was mainly 2D and was still played from a top-down perspective, thus it is still considered part of this universe. The London packs are the only ones to feature "real" cities. GTA 2 is set in Anywhere City, which does not appear in any other game in the franchise again.