The Old Town was 90% destroyed by these armies and by the Russian forces seeking to wrest Gdansk away from Nazi Germany. My understanding is that the sensitive restoration of the city after 1945 did reduce some of the Prussian influences in the architecture, but that most of the buildings looked exactly as they did before 1939.
Drawings, paintings, and old plans were used to reconstruct whole neighbourhoods, as was the case with most Polish cities after WWII, and as I walked along each street, strolled through the parks, and admired every church I gave thanks to the restorers for making such great efforts to reproduce their city of the pre-war years. It would have been easier to build Stalinist blocks, but the planners wanted their city back as it was in early 1939. It took over 30 years to complete, but the Old Town is now old again.