That's okay to buy the ticket the the entrance. If this is not the destination on your first day, you can buy it in advance.

If you buy the park ticket at Tung Chung City Gate Outlet shopping mall, you will have a extra gifts for each tickets you purchased.

That's more profitable for you, cause you can save your time on queuing up at the main entrance to buy ticket, and get a offical limited X'mas gifts.

The City Gate Outline in Tung Chung only 2 MTR stations apart from Disneyland Park, that's only take 15 minutes traffic time between 2 location. Also you do some shopping at that shopping mall, nearly all the shops having discount in X'mas time.

Hope the informations are useful for you.