To make it easier regarding the previous document, Geneva Convention and Treaty of Versailles :
People born in german territory will get the German nationality which will be preserved till they want to change by their own.
The Poland Republic didnt change all People nationality magically.
This is just like today:
You received your nationality at the moment of your birth depending on the territory controlled by the Autority. You can change nationality when your reach another country or territory.
If you lost the German nationality then the law must be respected. Either your GF changed it or your father. But you cannot lost the German nationality automatically. Its no a respect of the Geneva Convention.