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