From the API:
A GtkComboBoxEntry is a widget that allows the user to choose from a list of valid choices or enter a different value. It is very similar to a GtkComboBox, but it displays the selected value in an entry to allow modifying it.
In contrast to a GtkComboBox, the underlying model of a GtkComboBoxEntry must always have a text column (see gtk_combo_box_entry_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 GtkComboBoxEntry, as well as when selecting an item from the GtkComboBoxEntry'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 GtkEntry (use gtk_bin_get_child()) to detect when the user actually finishes entering text.
The convenience API to construct simple text-only GtkComboBoxes can also be used with GtkComboBoxEntrys which have been constructed with gtk_combo_box_entry_new_text().