As far as I know (and I've lurking on the gimp developer list for 10+ years), GIMP has never been a GNOME application. So, from that point of view, there's no particular reason to expect it to conform to GNOME's human-interface guidelines, is there?
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.
