If you need to solve a tricky GTK+ problem in your application, gtkparasite is a very useful tool to have around. It lets you explore the widget hierarchy, change properties, tweak theme settings, and so on.
Unfortunately, gtkparasite is a tool for people ‘in the know’ - it is not part of GTK+, not advertised on our website, and not available out of the box on your average GTK+ installation.
At the Developer Experience hackfest in Berlin a few weeks ago, the assembled GTK+ developers discussed fixing this situation by making an interactive debugger like gtkparasite part of GTK+ itself. This way, it will be available whenever you run a GTK+ application, and we can develop and improve the debugging tools alongside the toolkit.