More information about getting involved with other GNOME apps (and many tips that apply to GDM) can be found at http://live.gnome.org/GettingInvolved. For GDM internals refer to the GDM documentation.
Just as many other GNOME applications, GDM is translated by the GNOME translation team.
All icons drawn for GDM should match the GNOME Icon Style Guide
If you want to write documentation for GDM, you should join the GNOME Documentation team.