"Gail 1" is an original german scenario for the Cthulhu RPG published by Fantastische Spiele GbR and has later received a follow-up, "Gail 2".
From the back of the book (translated):
The desperate notary Clark Peterson hires a private investigator and his frieds to clear up the disappearance of his 18y old ward Mary Gail in wintery London of 1919. Soon they find out that this is not a normal case of kidnapping. A bloody trail leads in a chaotic dance of cosmic horror through the capital of the Empire and into an endlesse bog of madness, blasphemy and murder.