Arthur Duncan (husband) †
Barnabas Abernathy (husband) †
William Buccleigh MacKenzie (son)
Geillis Duncan is the wife of the procurator fiscal, Arthur Duncan, and lives in Cranesmuir, a village near Castle Leoch. She shares Claire's passion for herbs, though her interests extend beyond simple healing and remedies into much darker practices.
The following contains plot spoilers – read at your own risk.
Gillian was born in the 20th century and first married to Greg Edgars, a drunk. She believed she was a witch and changed her name to Geillis. After finding out about Craigh na Dun, she recorded detailed notes about how to get through it, researching people she believed tried to or successfully went through (including Claire Randall). Finally, she went through back to the 18th century; since she believed blood and fire were needed, she killed Greg before going through the portal.
Her obsession with the Jacobite cause was her chief motivation to travel back to the 18th century, to try and change the course of Scotland's history. She has a penchant for killing her husbands to inherit their wealth. She is a direct ancestor of Roger MacKenzie, her great-great-great-great-grandson.
She married the local fiscal Arthur Duncan and began stealing money from him for the Jacobite cause; she ran a herb store to the public. She started an affair with Dougal MacKenzie, the war-chieftain of the MacKenzies, in order to get into a more powerful positon. She also grew close to Claire Fraser due to their joint knowledge of herbs. Geillis killed her husband later by poisoning him; which lead to her gaining his money and belongings. When Geillis discovered she was pregnant with Dougal's child he informed his brother, the Cheiftain of the MacKenzies, Colum, who had her tried for being a witch along with Claire. She managed to save Claire's life by revealing her pregnancy and claiming Claire'a innocence but she was sentenced to be burnt to death after she gave birth to her child. When she did Dougal came to take their newborn son away until she threatened her child's life unless she was given her freedom. So Dougal took her to France for a new life.
After living in France (during that time she had two more husbands that she too killed) she traveled west to Jamaica where she married Barnabas Abernathy, a wealthy plantation owner; she killed him soon after their marriage. She then started to kidnap young white boys and hold them prisoner in her home, one of them was Ian and Jenny Murray's youngest son, Ian Jr. She would take each boy to her room, drug them and, if they were virgins, would search for a stone only virgins had. It's unknown what she wanted from them apart from the stone, but afterwards she would have them killed. Ian Murray was the last boy she took to her room; and since he wasn't a virgin she only slept with him but still kept him prisoner. Jamie and Claire Fraser, looking for Ian, came across her and found out her story while they searched for young Ian, but Geillis also found pictures of their daughter Brianna. She then hired a man to search through the Fraser history to find the descendants of Simon Fraser. When discovering Brianna was the last descendant she decides to travel back to the 20th century after finding out Claire had been back and forth three times but focusing on- in Claire's case- both her husbands. So she took Ian to a cave were there was a portal and started a ceremony she believed would help her get through in the future. But the Frasers stopped her before she could. She then held a gun to Ian's head threatening to kill him if they came any closer, but Jamie did and she shot him. This causes Claire to decapitate Geillis with an axe as Geillis aimed to use her daughter and her having just shot her husband. Her remains stayed in the cave until the 20th century where they were found by medical students and taken to the hospital that Claire worked at (before she returned to the 18th century) where Claire and another doctor attempted to identify her.