If you are going to use GTK+, take a look at Vala [1]. It's a C#-like language which is compiled into C. Much saner than coding in C. You can easily port your app to Windows/OSX, but, as others noted, non-native UI will be far from perfect.

I'm using this approach in an open-source project of mine [2], feel free to borrow the ideas.

[1] http://live.gnome.org/Vala

[2] http://www.spek-project.org/