Gull Identification
Hi Hendrix,
It looks big and solidly-built so I'd go for Heuglin's Gull ( which often has a black tail-bar in winter plumages).
Immature Black-tailed Gulls have a milky-coffee coloured appearance "body is washed brown rather than streaked..." (The birds of HK and South China, by Viney,Lam and Phillipps) .
Regards
John