You can move content area (GtkVBox) from GtkDialog to another GtkContainer, and then append to GtkNotebook - using gtk.Widget.reparent() method.
about = Gtk.AboutDialog()
about.set_program_name ("Application")
about.set_copyright ("Author")
box = Gtk.VBox ()
about.vbox.reparent (box)
notebook.append_page (box, Gtk.Label("About"))
about.destroy ()
With interface created in Glade, You must do simple trick for this solution. Create "About" tab in notebook with appended GtkBox (with id for example "about_box"). Then you can do something like this:
box = builder.get_object ("about_box")
about.vbox.reparent (box)
In some reasons Glade's GtkAboutDialog content area contains also GtkButtonBox. If You don't need this buttons simply destroy them:
about.action_area.destroy()
If You want only 'Credits' button, without 'Close' button, do something like this:
close_button = about.get_widget_for_response(Gtk.ResponseType.CANCEL)
close_button.destroy()