At the recent Kapow convention, he confirmed that the new run will be comprised of adaptations of 'A Scandal in Bohemia', 'The Hound of the Baskervilles' and 'The Final Problem'.
"Having had a successful first run, we knew that the natural order would be to do the three most famous things, so we're doing a version of [those three stories]," he explained. "They just fall like that, and then it's a question of how to get out of that cliffhanger!"
Gatiss explained that he will adapt 'Hound' for television, while co-creator Steven Moffat will write the series premiere.
"Steven wanted to do the twisted love story [of 'A Scandal'] because he likes that," he said. "I'm doing the gothic horror, and Steve Thompson is doing 'The Final Problem'.
"Now that the scripts are well underway, [it's] about trying to find certain things between the three of them, where we can find a little bit of an arc."
He added that the next three episodes will examine "what Sherlock and John's relationship is, a little further on".
"You don't want to just go back to the beginning," he claimed. "You've got to move on somewhere."
The second series of Sherlock is expected to air on BBC One in late 2011.