If you want to add new boards, download and run GCompris, it comes with a full documentation that describes the GCompris internal and a step by step instruction on how to add boards. To give an idea of the amount of work, a board is usually set in a single file of about 500 lines of C code or Python. GCcompris is based on the widget Canvas from Gnome). Have a look also at the API of the gnomecanvas.

Slide presentation of the GCompris architecture The last 4 slides give a good architecture overwiew.
  Full presentation in Openoffice.org format
  Full presentation exported in HTML .

Developers documentation for activities in python:
  Python wrapper for GCompris (by Olivier Samyn)
  Python Reference
  Python Tutorial

SVN allows you to access the latest source code

GCompris code is available from the GNOME Subversion repository.

From the Gnome web site:

Many people who are interested in working with GNOME development find their first hurdle is figuring out how to use Subversion. This document will be a quick tutorial.

Subversion is a powerful method of allowing many developers work on the same source code. This is possible because each developer checks out a copy of the current version of the source code. Then they each independently work on their personal copy of the sources. When they have made changes, they commit them back to the Subversion repository. The Subversion server takes care of things like trying to merge their changes with those of others. When that doesn’t work, the developer is notified and they do a hand merge of the conflicts.

To get the newest GCompris source from Subversion, you need to have it installed.

The first command you need is:

To read more on Subversion on Gnome go there.

Alternatively you can browse the GCompris code online.