GTK+ 3 Reference Manual
Windows
GtkDialog
— Create popup windows
GtkInvisible
GtkMessageDialog
— A convenient message window
GtkWindow
— Toplevel which can contain other widgets
GtkWindowGroup
GtkAboutDialog
— Display information about an application
GtkAssistant
— A widget used to guide users through multi-step operations
GtkOffscreenWindow
— A toplevel to manage offscreen rendering of child widgets