Gatiss, who found fame in British comedy 'The League Of Gentleman', is most well known for his role as Mycroft in BBC One's 'Sherlock' and also for his work as a writer on 'Doctor Who', reported Entertainment Weekly.
Gatiss' role is currently being kept secret.
The actor is appearing at this year's Comic-Con event in San Diego promoting the third series of 'Sherlock'.
It was previously confirmed that Pedro Pascal would be starring as Prince Oberyn 'The Red Viper' Martell in the show's fourth season.