In contrast to a Gtk_Combo_Box, the underlying model of a Gtk_Combo_Box_Entry must always have a text column (see Set_Text_Column), and the entry will show the content of the text column in the selected row. To get the text from the entry, use Gtk.Combo_Box.Get_Active_Text.
The changed signal will be emitted while typing into a Gtk_Combo_Box_Entry, as well as when selecting an item from the Gtk_Combo_Box_Entry's list. Use Gtk.Combo_Box.Get_Active or Gtk.Combo_Box.Get_Active_Iter to discover whether an item was actually selected from the list.
Connect to the activate signal of the Gtk_Entry (use Gtk.Bin.Get_Child) to detect when the user actually finishes entering text.
The convenience API to construct simple text-only Gtk_Combo_Box can also be used with Gtk_Combo_Box_Entry which have been constructed with Gtk_New_Text.