This seems like a fairly poor article overall... His point about GTK is nonsensical - C is a very good language to use because it means that bindings can be made for pretty much any language any language - which is why you can use GTK in any language from C++, to Python, to C# and Java, PHP, Javascript and so on... And C is used on thousands of projects more than 10K LOC, so I don't see how it's 'not suitable'...
What part of it is painful? I've found gobject to be quite effective. It's not the most enjoyable to extend (there's marshalling and other boilerplate that's not difficult but not interesting either), but in the majority of cases when using GTK you're doing just that--using it, rather than extending it. Using C avoids the complex semantics of C++ templates/inheritance at the cost of losing type safety when things become void*, but if you a have a little discipline that tradeoff grants flexibility that's quite nice in my opinion.
