I'm quite sure because I believe Grey-headed is the only gull with the colour of the head being almost the same as the colour of the back with the exception of some non-breeding species like Black-headed, but I think this individual has too much grey on the head to be one of those species in non-breeding plumage.
Also, this bird doesn't look like a juvenile to me.
looking around on Google, I found quite a few sites saying that Jurong has both Silver and Grey-headed Gulls on display (in the shorebird aviary - although this one looks like it is in the penguin house?).