Gtkparasite has long been available as a utility that's self-described as "similar to Firebug [an HTML inspector/debugger] but for GTK+ applications." A few weeks ago at the Developer Experience hack-fest, GNOME developers began to form this new GtkInspector utility that's derived from the independent gtkparasite project.
After work in the past few weeks by Matthias Clasen, GtkInspector has now landed within the GTK+ code-base and will be part of the GTK+ 3.14 release. GtkInspector will now be available whenever running a GTK+ application, can be activated by the Control-Shift-I or Control-Shift-D shortcuts, and allows inspecting of individual widgets, visual debugging of graphic updates, changing of widget properties, tweaking the GTK theme, and other abilities.
Those wishing to learn much more about GtkInspector and what it offers GTK+ developers, visit Clasen's blog where he also has a video and other resources on this new GTK+ component.