History of Gloucestershire
The region now known as Gloucestershire was originally inhabited by Brythonic peoples (ancestors of the Welsh and English and other Romano-British peoples) in the Iron Age and Roman periods. After the Romans left Britain in the early 5th century, the Brythons re-established control but the territorial divisions for the post-Roman period are uncertain.
Source: Wikipedia — History of Gloucestershire (CC BY-SA 4.0)