@Efreet: could you please explain how you came to your conclusion? I mean fine you answered the topic title question, but the first post has more questions to answer. (But I can understand why you don't if your first language is not English.)
The writer behind the game, Dan Houser, stated in several interviews that GTA is America. Looking at it from another angle, it wouldn't make sense to create a whole-world map as a playground for a video game. The budget for such a game would be insane, let alone making it to match R*'s level of realism and details. It's a silly and unnecessary idea.