I'm not sure, but I think these are new Bus Gates. The GAT in DUS is called "Modul D". And they built lot's of new parking positions for smaller planes in front of it last year. Pretty close to this new parking positions are the old parking positions, for example V73. So maybe they re-named the parking positions around the "Modul D" as D-Gates!? That would explane a D73 gate. Do you have an example for flights flying into these gates?