Why would this not be Whiskered Tern? It's the commonest tern wintering in southern India according to my India books. It's the right colour for winter plumage - black bill with reddish legs and a grey/black cap. (Little has yellow legs and Saunder's brown, I think). It's small (23-29 cm compared with 22-28 for Little a/c Mark Brazil's book) and has a short tail.
White-winged Tern would also be a possibility according to the books, but I think this has a bigger black 'blob' behind the eye, and I think the legs don't show red like this.
The legs are wrong for Gull-billed, and it would look much chunkier, I think.
Some Whiskered Terns passed through my patch in central Japan last 1st October presumably blown off course by a major typhoon. They stayed on our pond for about ten days. (Not many pass through Japan, and then normally along the west coast, a long way away: scarce in Autumn, rare in Spring a/c Brazil; this was the first time for me to see them.)
The poster's bird seems to have a slightly larger bill, and a little less colour in the wings than the birds I saw, but but I don't see why it wouldn't be in the range of variation.
Then last weekend, on 31st March, we found that three of them had come to visit us on their way back home again! Exactly six months later.