AFAIK GtkDropDown does not offer free text entry and is not a 1:1 replacement for GtkComboBox, but rather meant to really select one entry from a list of provided options. I would work around by providing a "Custom"/"Other" etc. option in the dropdown and use the callback method to unhide or enable a separate GtkEntry. Even if it seems bulkier at first, it uses basic user elements that exist in any UI framework rather than specialized ones of Gtk.