I took a quick look at PyGObject[0] (unsure if that is what one would actually use for this), and the most helpful part seems to be [1], giving some hints on extending GObject.Object, which should be translatable to extending other existing GObject type classes.
As for porting, I agree that can be a pain in that case. When bindings move from a manual implementation to inflating it from an external description. You often end up with good tutorials for the first, and good API reference for the second. If I need to understand other kinds of Gtk bindings, I search for examples on custom TreeModels or other potentially messy things like that to get a feel for it.
[0] https://wiki.gnome.org/action/show/Projects/PyGObject?action... [1] http://python-gtk-3-tutorial.readthedocs.org/en/latest/objec...