Fortunately for Dailly, the original GTA world map was created in 3D. Most players would have missed this due to the game's 2D aesthetic, but a proper 3D map was the simplest method for the developers to show height in the game's buildings, and the only way to allow players to traverse bridges and tunnels. Its even possible to alter the game's code and play through the original GTA from a ground-level perspective, not unlike more recent entries in the franchise.
Before you get your hopes up for a new, fully-fledged GTA entry hot on the heels of Grand Theft Auto 5, it should be mentioned that Dailly currently has no plans to implement Grand Theft Auto-style gameplay within his game, nor any definitive idea of what he'll do with this project once it's complete. Instead, his goal appears to be a simple proof of concept. As head of development at YoYo Games, Dailly has been using the company's flagship GameMaker software to rebuild the GTA engine, and if nothing else he hopes his remake will serve as evidence of what's possible with YoYo's software.