I looked up Julius Gerson at https://www.mappingthelives.org
and it shows that he was born at Bischofsburg in 1899, but was living at Koenigsberg in 1939.
So "Bischofsburg-Koenigsberg" is two places, not one.
Bischofsburg, now known as Biskupiec was the town where my great-grandfather's siblings were all born. It was in Kreis Roessel (Rößel) but is now in Powiat Olsztynski (Allenstein)
By the way, mappingthelives shows that Erna Czerninski née Gerson as well as Jacob and Manfred Czerninski and Käte Bohm lived at the same address as Julius in 1939. Erna was born at Nikolaiken in 1895.
I hope this helps,