Yes, I found a site that contains documentations for all python bindings of perhaps all GNOME projects that are supported by the python gi.repository module or the Python GObject Introspection module:
http://lazka.github.io/pgi-docs/
The one specifically for GTK is:http://lazka.github.io/pgi-docs/Gtk-3.0/index.html
There is also documentation specific to the gi package provided by PyGObject:
https://pygobject.readthedocs.io/en/latest/guide/api/api.html
I hope you find these links helpful