Unfortunately, there are no GUI tools for point-and-click editing of the GDM Theme as of yet.
However, the official GNOME documentation explains the different parts of the GDM theme. Its biggest problem though is its lack of visuals to explain the content.
You can find the documentation here:
Official GNOME Documentation
PS. I am also in the process of writing a beginner's tutorial/manual for GDM theming (with pictures).
I will give you the link as soon as I find that it is ready.