The name Gillian has its origins in the medieval French form of the ancient Germanic name Gilli, which means "youthful" or "young." It is derived from the Old French name Gile, which was a diminutive of Guillaume, the French form of the Germanic name William.

The name Gillian first gained popularity in England during the Middle Ages. It was initially more commonly used as a masculine name, but by the 13th century, it had also become a feminine name. The earliest recorded use of the name Gillian as a feminine name dates back to around 1200 AD.

In medieval literature, one of the earliest known references to the name Gillian can be found in the 12th-century French romance "Le Roman de la Rose," where it is used as a masculine name. Another early reference is in the 14th-century English poem "The Canterbury Tales" by Geoffrey Chaucer, where the name Gillian is used for a female character.

Throughout history, there have been several notable individuals who bore the name Gillian. One of the earliest recorded was Gillian de Vaux, a 12th-century English noblewoman who was the mother of Hubert de Burgh, a prominent figure in the reign of King John. Another early bearer of the name was Gillian of Norwich (c. 1366 - c. 1430), an English mystic and author of the first known book written in English by a woman.

In more recent times, the name Gillian has been borne by several famous individuals, including Gillian Anderson (born 1968), the American actress known for her role as Dana Scully in the TV series "The X-Files," and Gillian Jacobs (born 1982), an American actress best known for her role as Britta Perry in the sitcom "Community."

Other notable bearers of the name include Gillian Barre (1874-1959), a French neurologist who first described the condition now known as Guillain-Barré syndrome, and Gillian Tett (born 1967), a British author and journalist who has written extensively on financial markets and economic issues.