First off, you probably want to use GtkBox with a center widget instead
of this widget. However, the case where this widget is useful is when
you cannot control your layout within the width of the toplevel, but
still want your child centered within the toplevel.
This is done by translating coordinates of the widget with respect to
the toplevel and anchoring the child at TRUE_CENTER-(alloc.width/2).
First off, you probably want to use GtkBox with a center widget instead of this widget. However, the case where this widget is useful is when you cannot control your layout within the width of the toplevel, but still want your child centered within the toplevel.
This is done by translating coordinates of the widget with respect to the toplevel and anchoring the child at TRUE_CENTER-(alloc.width/2).