Former Neighbours star Little is moving on to the Finborough to star in The One Day of the Year.
Gallagher is best known for his recent role as Caiaphas in the arena tour of Jesus Christ Superstar with Tim Minchin.
Other notable roles include Miles Gloriosus in A Funny Thing Happened on the Way to the Forum (Regent's Park) and Big Jule in Guys and Dolls (Sheffield Crucible).
He returns to the world of Rocky Horror having previously played Eddie/Dr Scott in the West End.
Shock Treatment opened for a three week run on 17 April, marking the show's stage premiere.
The second three week run starts on 19 May with returning original cast members Julie Atherton and Ben Kerr as Janet and Brad, Rosanna Hyland and Mateo Oxley as Betty and Ralph Hapschatt, and the Twins Macabre, Adam Rhys-Davis and Nic Lamon as Cosmo and Nation McKinley.