I got this from https://wiki.gnome.org/Projects/GTK%2B/Inspector:
GtkInspector is the built-in interactive debugging support in GTK+. It was added in GTK+ 3.14, based on a copy of the well-estabished gtkparasite.
The debugger is disabled by default. To enable it run in a terminal:
gsettings set org.gtk.Settings.Debug enable-inspector-keybinding true
And launch it pressing on the keyboard Control-Shift-I or Control-Shift-D.
If you don't want to use the shortcuts, you can also run it temporarily directly when running your app with:
Looking further, I found Bug #1523929 which indicates that users can install
libgtk-3-dev which then provides the schema. Note that installing
libgtk-3-dev may bring in a lot of dependencies.