The name Greg is a diminutive or shortened form of the ancient Greek name Gregorios. Gregorios itself derives from the Greek word "gregorein" meaning "to watch" or "to be awake." The name likely originated in the 3rd century AD among early Christian communities in the Byzantine Empire.

Gregorios first appears in records as the name of a few early Christian saints and martyrs from the 3rd and 4th centuries. The most famous of these was Saint Gregory the Illuminator, the first official head of the Armenian Apostolic Church in the early 4th century. Another noteworthy early bearer was Gregory of Nazianzus, a influential 4th century archbishop known as "the Theologian."

The shortened form Greg emerged in the Middle Ages and grew popular across Christian Europe. One of the earliest examples is Pope Gregory I, also known as Gregory the Great, who reigned from 590-604 AD and was an extremely influential leader of the early medieval church.

Other historically significant individuals named Greg include the 11th century German monk and scholar Gregory of Tours, Gregory VII who was pope from 1073-1085, and Gregory IX who served as pope from 1227-1241. In England, Gregory the Great's emissaries helped spread Christianity there in the late 6th century.

As the name disseminated across the continent, variant spellings emerged like Gregor, Grégo, and Grégoire, reflecting linguistic differences across regions. Gregor Mendel, the 19th century Augustinian friar and founder of modern genetics, was born in 1822. More recently, Pope Gregory XVII led the Catholic Church from 1959-1963.

While common today, Greg retains its historic connections to early Christianity and the noteworthy individuals, both clergy and laypeople, who advanced religion, scholarship, and exploration over the centuries. The name remains popular among communities with Greek or Christian cultural roots.