is GD library necessary to run Drupal ?

But if you would want to use some image handler module like ImageCache, it will be necessary.

If you're not able (or don't prefer) to setup a full server environment yourself on your local system, you should get one of the excellent pre-packaged solutions. For you, MAMP (which is what I and many people use): http://www.lullabot.com/videocast/install_local_web_server_mac_osx