I had the same problem. I didn't much look into customizing it in Gnome, because I knew I liked the KDM login screen. If you install KDM, you can set that as your desktop manager, and then download themes to use with it or create your own theme. It's been a while since I did this, so I can't remember the exact commands, but I'll pull up my desktop on my phone and figure out what they are.
Okay, you'll need to run
sudo apt-get groupinstall "KDE Software Compilation" to install KDE. This will install the KDM you want to use, as well as allow you to load KDE to chenge the KDM options. Then you'll need to edit
/etc/sysconfig/desktop (or create the file if it's not there) to include the lines
Then reboot, and it'll use KDM instead of GDM! The first time you do it, you may have to tell it to use Gnome instead of KDE. I can't remember whether I had to or not. You can download KDM themes here, and then load KDE to change which one you are using.
Please see my comment for another method which I haven't tested, but is probably better.