The GTK+ 3 Tutorial is an introductory guide through the widgets available in the GTK+ 3 toolkit, combined with the use of the C programming language.
The tutorial does not guide through the building process of an application from start to finish, but rather provides the common methods and signals used by each widget, and a description of how each works.
The main GitHub repository for this documentation is https://github.com/Programmica/gtk3-tutorial.