Galen was Human male of Greek origin who lived on Earth during the 2nd and 3rd centuries A.D.. He was named among the inspirational medical practitioners from among several Starfleet doctors, their patients, and medical personnel in historical and literary contexts, which were included in the Emergency Medical Hologram programming.
When Kes suggested that The Doctor could name himself after someone who inspired him, he mentioned Dr. Galen, Dr. Salk and Dr. Spock. None of those names, however, were ones The Doctor could see himself with. (VOY: "Ex Post Facto")