This is not an entirely fare comparison. I suggest comparing Qt to Gtkmm which is the official C++ wrapper of GTK+.

To get a better perspective, please, refer to this discussion date back to 2004.

Back in the day, GTK+/Gtkmm came to existence to counter-balance success of Qt as multi-platform GUI toolkit. At that time Qt was not an open-source solution at least on Windows. So, GTK+ was a solution to an ethical problem related to Linux desktop (R. Stallman on GTK+ vs. Qt dilemma).

Here are some pointers:

GTK+
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1. GTK+ is favored by those C developers who need speed or find themselves strangled in an embedded world. In addition, most of electrical engineers who never had a chance to learn OOD with C++/Java/C# choose GTK+ to write GUIs for embedded platforms.

2. GTK+ is well documented with number of books published.

3. There is virtually 0 jobs that require GTK+. So, to me it seems that GTK+ is mostly used by free software developers or in-house jobs.

Gtkmm
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1. Favored by C++ developers who wanted a guarantee from day one that the framework is open-sourced across Unix/Win/Embedded platforms.

2. Aside from user's guide for beginners and developer's mailing list, there is no documentation touching more complex aspects of s/w development. This is hacker's nirvana where you derive masochistic pleasure of debugging both GTK+ and Gtkmm with gdb to figure out why your tiny app crashes all of the sudden.

3. Same as for GTK+, there is no commercial market for Gtkmm developers. So, it is a hobby rather than a career.

Qt
===

1. A commercially-supported multi-platform C++ GUI toolkit long before Java/C#/Python gained the ground.

2. Well-documented from early days on.

3. There is some commercial work with Qt and hopefully with Nokia throwing its weight behind Qt across their fleet of mobile devices, there would be more of job offerings with Qt required.

So, GTK+, Gtkmm, and Qt appeal to 3 vastly different segments of programmers.

It would be really nice if Hildon were supported by next Nokia MeGo phone. Otherwise, developers of a whole bunch of Gtk+/Gtkmm embedded applications face a choice to either

1) abandon MeGo altogether or

2) abandon Gtk+/Gtkmm, bite the bullet and move on to Qt.