Doctor J. Gilnitz was a staff doctor at St. Clair Children's Center.
She worked with William Vinet on Emily Christine Sim, and tried to take a biopsy of the growth on the back of her neck. Despite Fox Mulder's protest, she started to take the biopsy but, not realizing its toxic nature, was quickly harmed by the outgassing of the blood inside. (TXF: "Emily")
The character's name comes from her name tag as she tended to Emily. The name is another instance of John Gillnitz.