Talking to Entertainment Weekly, the showrunner refused to divulge details about the episode's plot but promised that fans would not be disappointed when it finally airs.
"It's called 'The Doctor's Wife'. I can't say very much actually because the whole gimmick behind that show - and it's a very clever gimmick - will be unveiled in the opening minutes," he explained.
"I literally can't talk about it without giving it away, which I don't want to do. It's a lovely, magical, mad episode."
Asked if he felt that Gaiman struggled to adapt to Doctor Who's unique scripts, Moffat answered: "He would bring his own estimate of that. I think everybody - and I don't think Neil would argue with this - everybody who comes onto the show gets a fright at just how difficult it is.
"You assume you know it, until you try to write it. Then you realise it really is a monster. I've written an awful lot of them now and I still find it shockingly difficult. I think everyone who comes on has that moment of 'I didn't think I'd be working this hard'."
Gaiman previously joked that writing an episode of Doctor Who made him feel like God.
The Good Omens author has also said that he is happy with how his Who episode turned out.
Doctor Who airs Saturdays on BBC One in the UK and BBC America in the US.