Gillian is the medieval English feminine of Julian — from the Latin Iuliana ("youthful, of Jupiter"). Sex Education (Netflix 2019-2023) as Jean Milburn. Gillian Jacobs (born 1982) — Community (2009-2015). Gillian Welch — American Grammy-winning roots musician.
Gillian does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Gillian reduce to 1, The Leader. This is a traditional interpretive system, not a factual claim about the name.