Gibbons say "they are really not canon."

Page from Watchmen. Art by Dave Gibbons.

[...] "as far as I’m concerned, what Alan and I did was the Watchmen graphic novel and a couple of illustrations that came out at the same time. Everything else – the movie, the game, the [laugh] prequels – are really not canon. They’re subsidiary. They’re not really Watchmen. They’re just something different." Dave Gibbons. 

You can read about the interview here. And you can see Gibbons talking at length in the video below. The Watchmen statement is about one hour seven minutes in.