This document, like so much other great software out there, was created for free by volunteers. If you are at all knowledgeable about any aspect of GTK that does not already have documentation, please consider contributing to this document.
I have no doubts that there are parts this tutorial that could be improved upon if I only had more time. Also, be aware that the entirety of this document is free, and any addition you provide must also be free. That is, people may use any portion of your examples in their programs, and copies of this document may be distributed at will, etc.