These all require authors - both for the C version and for Pascal! :) Please consider contributing to these tutorials.
If you must use one of these widgets that are undocumented, I strongly suggest you take a look at their respective C header files in the GTK distribution. GTK's function names are very descriptive. Once you have an understanding of how things work, it's not difficult to figure out how to use a widget simply by looking at its function declarations. This, along with a few examples from others' code, and it should be no problem.
When you do come to understand all the functions of a new undocumented widget, please consider writing a tutorial on it so others may benefit from your time.