Danzig's population was always to over 98% German
Have you ever heard about Pomeranians?
Whether they had any Polish blood is anybody's guess, but if he had, we'd probably know.
I don't know if i have polish blood. I never heard about genes defining Poles(I suspect that Hitler could have said something like that). I thought that being a Pole is something related to mother language, traditions, self-identification, living space and citizenship.
He was from Gdansk, but his family were culturally German.
So he was probably German.