I understand GObject started out as part of gtk+ and was later separated from the GUI related elements. What I would like to know is: it used outside gtk+ ? what about other desktop environments, and other OSs like Windows or Mac OS? Are there any prominent examples of such cases?
There are some things here and there that use GObject without GTK+, but AFAIK they are few and far between.
I'm guessing that the most prominent ones right now are Clutter-based projects (it's a graphics-oriented UI library). There are also a small number of projects based on libgnt (text-based UI library), and possibly various non-GTK+ programs written in Vala (a C#-like programming language with GObject-based classes).
Edit: Also GStreamer (thanks liberforce!), which is a popular multimedia library. The vast majority of GStreamer projects also use GTK+, but I'm sure there are some non-GTK+ ones.