I believe this is a first calendar year Heuglin's Gull, the most numerous large gull in HK.
Being so early in the winter, the bill is all dark and the bird is in fairly fresh plumage. The underparts are rather more heavily-marked than we are used to seeing in late winter, when these birds are most abundant in HK, but at the latter time a lot of bleaching has occurred.
The tail pattern comprises a broad dark terminal tail band, which suggests Slaty-backed, but the amount of white still visible at the base fits better Heuglin's.
Vega Gull is ruled out due to the overall darkness of the bird, the lack of any hint of paler inner primaries (which, admittedly, might be difficult to see on such views as these), and the breadth of the dark tail band.
Geoff Carey
