It uses the same crappy GTK file mgt widget design philosophy as GNOME.

http://en.wikipedia.org/wiki/GTK%2B GTK is the GIMP toolkit, and GTK is run by the GNOME foundation. There's quite a bit of relationship between GTK/GIMP/GNOME, even if it may be mostly historical these days.

I've always considered GIMP to be a GNOME app, and most people I know have.