Normally I read and learn when it comes to large gulls, but: are you both sure with Great-Black-backed Gull?
With such a pale eye in an immature bird, and the bill is not hefty enough, I think its a Herring Gull.
In immature GBB with the palest eyes, they are still amber coloured (allways?).
I made a similar misidentification before, and was kindly corrected by Steve Klasan https://flic.kr/s/aHskAsUjJx
With such plain dark underwings its allways good to consider a Yellow-Legged Gull, at least in S-Germany, but otherwise it doesnt strike me as one (among others quite uniform streaked head) and such an underwing is easy within variation for Herring Gull. But I am no expert and would like to learn here, if this is no Herring Gull, because I think, it is one.