I think Gtk documentation is not easy to use, specially for french people like me. In this repository I store different programs I've written for examples. Normally the documentation is inside but I can resume each here.
It was difficult to set a countdonwn and specially to modifiy duration or stop the timeout. This programm (anns associated css) is a good example to manage timeout in GTK.
A simple and beautiful scale with values display
The same with get button
A hamburger menu in the window barr
How to resize dinamicaly the type and size of font
Menus in menus