Ubuntu does come with a nice looking Log-in GUI/s for the GDM, but the more you use the same themes the more bored you get. So, out of your boredom, if you're searching for a cool looking GDM theme then the Willwill's Intrepid is a pretty good theme.
Unlike with other GNU Linux distributions, I think the developer has made this GDM theme for Ubuntu OS (you know when considering its color and stuff). So if you use Ubuntu and need a 3D looking theme for the GDM ... then first,
1. Download the package from here.
2. Then extract the content.
3. Move the extracted folder to = /usr/share/gdm/themes
Or you can open the GDM configuration window in Ubuntu and simply drag - n- drop the tar.gz file and it'll automatically install it. Now you should be able to apply the theme in your GDM configuration window. Enjoy!.