This repo followes this tutorial [1] to specify order of topcis and gets examples from there. Examples are same with there but expressions of topics are not.
You can see example codes in source directory, they are showed in topics as well.
Wiki page includes tutorial: https://github.com/COMU/rubygtk3/wiki/Ruby-Gtk3-Tutorial
For more ruby-gtk3 examples see this repository [2]. I have ported gtk2 examples to gtk3 which are gotten from zetcode. However the repo does not include explanation of topics, only shows examples.
[1] https://python-gtk-3-tutorial.readthedocs.org/en/latest/
[2] https://github.com/ebruAkagunduz/gtk3-ruby-examples