Moir - who is also known by his stage name Vic Reeves - will appear in the third episode of the drama's eighth series as a shady pawnbroker.
Meanwhile, Jody Latham (Shameless) will play Jonjo Burdon, described as a "skin head maniac", in the fourth and final episode of the series.
Inspector George Gently's eighth series will air on BBC One in early 2015, and is comprised of four feature-length episodes.
Sherlock's Louise Brealey, Emma Cunniffe, Anthony Flanagan, Adrian Bower, Paul Brennen, Danny Cunningham and Lucian Msamati will also feature in the new episodes.
Martin Shaw and Lee Ingleby will return for the new series, which is set in Durham in 1969.
Written by Peter Flannery and based on Alan Hunter's novels, the drama follows the titular London inspector solving crimes in the north east of England.