According to the document your grandfather was german and remains german althought some part of Upper Silesia was managed by polish autority. This period 1919-1924 was really a mess. The only exception is the fact that he changed his nationality by its own. Such a change must be done in any civilian polish state. So the easier way is to read the birth’s record and the marriage’s record of your GF. Can you post them here ?
By the way my Grandfather has changed its nationality when he left the Poland and this information was writting in its marriage’s record.