He was known for his horror movies, especially those he made for Hammer Films, for whom he directed The Shadow of the Cat (1961), The Plague of the Zombies (1966), The Reptile (1966) and The Mummy's Shroud (1967).

[1] He entered the British film industry immediately as an editor and assistant director, starting with Father O'Flynn.

[citation needed] After the war, Gilling wrote the script for Black Memory (1947), and made his directing debut with Escape from Broadmoor (1948).

Gilling continued through the 1950s making second features such as The Voice of Merrill for Monty Berman's Tempean Films and entered television directing in several British series that received international distribution such as Douglas Fairbanks, Jr., Presents and Gideon's Way, as well as Monty Berman's The Saint, The Champions, and Department S. Of his films for Tempean, the film historians Steve Chibnall and Brian McFarlane wrote: "Gilling shows in all of them a capacity for establishing the premises of his plots economically and evocatively, for developing them with clarity and speed, for giving competent players a chance to invest their characters with a feeling and detail that go beyond stereotype, and for making deft use of limited locations and settings".

Perhaps his very best film as a director is The Flesh and the Fiends (1959), the story of Dr. Robert Knox and the West Port murders, which starred Peter Cushing and Donald Pleasence.