The only thing which remains to tune my Gentoo up to resemble like a MacOSX enviroment was the Splash screen. Searching in GnomeLook doesn't solve the problem, but I found a GDM theme OSX-like [http://www.gnome-look.org/content/show.php?content=20896] (thanks Senki for the marvelous GDM theme!) so I decided to take parts of the screenshot file to make the splash screen, and this is the result.
To change the splash screen just put de image wherever you want it, then go to the gconf and go to apps > gnome-session > options > splash_image; there just simlpy change the key with the entire path of your image, and you're done!
If you want to put the image in user-protected folder, login as root and put de image in /usr/share/pixmaps/splash/, in this case you just have to put splash/image_name.png in the gconf key path.