In GTK+ a "drawable" is anything that you can "draw" to. This includes
windows (GdkWindow) and offscreen buffers (GdkPixmap, GdkBitmap).
Almost all the Gtk widgets handle their own drawing and need no help from you.
Most of them have an associated GdkWindow which they use for this purpose;
some, however, use their parent widget's window. So, trying to draw to a
window of a widget is futile since it will all be erased once the widget is
refreshed. If you want to draw your own little piece of art, use
GtkDrawingArea widget which gives you a blank canvas to start from.
Normally, you wouldn't draw to the GtkDrawingArea directly, because it doesn't
have any "memory" -- as soon as another window covers it, the content of the
underlying window is gone. What you have to do instead is draw to an offscreen
pixmap (GdkPixmap) and then copy the contents of the pixmap to the window
anytime it's exposed ("blit it").