gtk-3-examples software package provides example files for GTK+ 3, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install gtk-3-examples
gtk-3-examples is installed in your system.
Make ensure the gtk-3-examples package were installed using the commands given below,
$ sudo dpkg-query -l | grep gtk-3-examples *
You will get with gtk-3-examples package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | X Window System software |
| Package | gtk-3-examples |
| Version | 3.22.2-1 |
| Description | example files for GTK+ 3 |
| Sub Section | x11 |