She is portrayed by Lotte Verbeek.
Born in the 20th century, Gillian met Greg Edgars through the Scottish National Party, and they later married. Eventually, Gillian's knowledge of Scottish history evolved into an interest with the ancient stones of Craigh na Dun. Upon further research, Gillian came to believe that a ritual and sacrifice were needed to travel through the stones, so she killed her husband. Successfully travelling through the stones, Gillian ended up in the 18th century and went by the name "Geillis", later marrying procurator fiscal Arthur Duncan. However, she also has an affair with Dougal MacKenzie and becomes pregnant with his child.
In 1743, Geillis meets and befriends an Englishwoman named Claire Beauchamp, who also shares her knowledge of herbs and healing. Geillis is curious of who Claire really is and where she came from. During a feast at Castle Leoch, Geillis's husband Arthur suddenly dies, having seemingly choked to death. Some time later, Geillis and Claire are both arrested and are put on trial where they are accused of being witches. In order to have Claire spared, Geillis confesses to the court that she is a witch and that she murdered her husband, before going on to show the crowd a mark on her arm which she claims is the mark of the Devil, but Claire recognises it as a smallpox vaccination and realises that Geillis is also from the future. Claire escapes while Geillis is sentenced to death by burning at the stake.
Dragonfly in Amber
Having returned to the 20th century, Claire discovers that Gillian Edgars – a fervent student of history and member of the Society of the White Rose – is actually Geillis before she travelled in time in 1968. Claire, as well as her daughter Brianna and their friend Roger, discover Gillian's plans to enact a ritual to travel through the stones of Craigh na Dun. They go there in an attempt to stop her, since Claire knows that Gillian/Geillis is destined to be sentenced to death for witchcraft. However, when they arrive at Craigh na Dun, Gillian has already murdered her husband and manages to travel through the stones.
Earlier in 1968, Claire and her colleague Joe Abernathy examine the skull of a white woman found in a cave in the Caribbean. Claire gets the sense that the woman had been murdered.
After using the stones, Claire travels to 1766, where Jamie's nephew Young Ian is kidnapped by pirates while attempting to retrieve the treasure from the Selkie's Isle, with both Ian and the treasure taken to Jamaica. There, Ian meets Mrs. Abernathy of Rose Hall – who turns out to be Geillis.
Later, Geillis and Claire meet again, although Claire had believed Geillis to be dead. Geillis reveals that, since she was pregnant, her death sentence was postponed but after she gave birth, her lover Dougal MacKenzie arranged for her escape. Fleeing to Jamaica, Geillis married Mr. Abernathy but soon murdered him and inherited Rose Hall for herself. Claire explains to Geillis that she and Jamie are searching for Young Ian. Geillis feigns ignorance and concern, and offers to help her, but Claire later finds out that Geillis has kidnapped him. Geillis, having learned about Claire's daughter Brianna, believes that Brianna is the subject of a prophecy regarding the future of Scotland and takes Young Ian to Abandawe – a cave which also serves as a time portal, similar to the stones of Craigh na Dun. Geillis plans to go back to the future and find Brianna, but Claire and Jamie pursue them to rescue Ian. In a desperate attempt to stop Geillis from going after her daughter, Claire swings an axe at Geillis, practically decapitating her. Claire, overwhelmed by her actions, recalls the skull she examined back in 1968 – the skull of a white woman murdered in a cave of the Caribbean.
Written in My Own Heart's Blood
In 1739, Geillis's son Buck, and his time-travelling descendant Roger MacKenzie, goes to visit Geillis in Cranesmuir. Geillis is revealed to be having an affair with Dr. Hector McEwan and had been pregnant with his child before she had an abortion.
- Gillian/Geillis is extremely knowledge about plants and healing, as well as Scottish history, especially concerning the Stuarts.
- While Geillis incorporated a blood sacrifice when she travelled through the stones, Claire managed to travel back in time without sacrificing anybody.
- In Jamaica, Geillis is known by slaves as the bakra, and bathes in goats' blood.