The docudrama focuses on the making of the first series of the sci-fi, and coincides with the show's 50th anniversary.
Speaking to the Radio Times, Gatiss revealed: "I've been trying to do this [project] for years - I tried to do it for the 40th anniversary! And before that, really - it's taken about 12 years."
Harry Potter actor David Bradley has been cast in the role of First Doctor William Hartnell, and Gatiss admitted that he has had the actor in mind for the part for "years".
The Sherlock writer said of the project: "It's a real labour of love for me and I'm just really thrilled that everything's come together for the 50th.
"It just seems exactly the right time, and a wonderful cast and it's a very lovely story."
[David Bradley in previous 'Doctor Who' episode 'Dinosaurs on a Spaceship']
Gatiss has also written for the upcoming eight-episode run of Who, which will begin in March on BBC One and BBC America.
Showrunner Steven Moffat has said that he is "mostly excited" for the 50th anniversary but also "a little bit nervous and aware of trying not to let people down".
Watch the stars and producers of Doctor Who discuss the show's 50th anniversary with Digital Spy below: