Gtk::Object is the base class for all widgets, and for a few non-widget objects such as Gtk::Adjustment.
Gtk::Object predates Glib::Object; non-widgets that derive from Gtk::Object rather than Glib::Object do so for backward compatibility reasons.
The most interesting difference between Gtk::Object and Glib::Object is the ability to use Gtk::manage() to delegate memory management to the container widget. Gtk::Objects can also be explicitly deleted at any time, instead of using only reference-counting, and container widgets can respond when their child objects are being deleted (for instance by removing the widget).