i think you need to work on your models, try editing it in a 3d software(3D studio max would be great) and import it to med as assets for your game.

I think it's because your trying to imitate GTA that's why you want it to look like a real GTA game compromising your game visuals but besides that, the game is freaking awesome. I was so amazed when I saw the entire level like a small city...gees men your a legend!