Gnome icons on the desktop are managed by Nautilus (the Gnome file manager). Either you use nautilus on fluxbox or you have to install any kind of icon management system like adesklet for example.
Fluxbox does not have icons on the desktop by itself.
Or do you run Gnome with fluxbox together?
Than you simply add with right-click on the desktop a "new launcher" and add the command aterm -whatever and choose an icon.