Yes, clearly a Greylag.
...but that has to be one of the most obvious cage damaged birds I've seen. The hkwildlife pictures show there are no feathers on the back of the neck, vitually no tail, and severe damage to the whole of the culmen (top of the bill).
Greylag Goose is long overdue in HK, but I don't think this one is going to get past the RC as a genuinely wild bird!