Hoho, at the moment, there is no such thing call water proof plaster ceiling, which may be have just i never hear yet. by the way, upstair are right, since is in door ceiling, what so worry about raining? the ceiling may spoil becoz of raining is the problem of leaking on your roof, so if there is a leaking, just ask sometime to fix the leaking part, then will be no problem already.

Note: other then plaster ceiling, any kind of ceiling also will have problem if roof leaking, so i don think you need to worry about plaster ceiling.

Gypsum board, actually is just 2 plaster (which same as plaster ceiling) pressed between two thick sheets of paper. the only diff is gypsum have additional 2 pcs of paper and material to do plaster ceiling dont have. so it nothing much diff if the roof is leaking.