Heike, although I sent you a PM with lots of info on the Via Gebensis the other day, I think I omitted to mention anything about the credencial.
If you are not able to get a credencial in time - I think you are leaving today - you can make one yourself to have stamped along the route. When it comes to staying in the albergues in Spain and, especially collecting stamps on the last 100km from Santiago, that is when you need an accepted credencial.
Have a wonderful walk.
Sil