Here are the classes, structs, unions and interfaces with brief descriptions:

Glib::Util::BadConversion	Conversion exception
ColumnRecord
EntryCompletion
Gtk::Util::EntryMultiCompletion
Gtk::Util::EntryMultiCompletionModelColumns
EventBox
Glib::Util::Exception	Base exception class
Glib::Util::Initializer	Initialization worker, necessary if you need thread-safety
Glib::Util::LogStream	The log stream class
Glib::Util::OptionDesc
Gtk::Util::PageNavigator	Navigation bar
Glib::Util::ScopeLogger
Gtk::Util::Tile	Extensible widget providing a highlighted title, summary and an image on the left-hand side
Gtk::Util::TileView	The Tile container
VBox
Gtk::Util::WhiteBox	A custom Gtk::EventBox containing a Gtk::VBox painted white