Gallo-Brittonic is a term used to describe the common aspects between Celtic Gaul

Gaul

Gaul was a region of Western Europe during the Iron Age and Roman era, encompassing present day France, Luxembourg and Belgium, most of Switzerland, the western part of Northern Italy, as well as the parts of the Netherlands and Germany on the left bank of the Rhine. The Gauls were the speakers of...

 and Celtic Britain

Great Britain

Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...

. These include linguistic and other cultural features, as well as history. The cultural aspects are commonality of art styles and worship of similar gods. The coinage just prior to the British Roman period was also similar. In Julius Caesar

Julius Caesar

Gaius Julius Caesar was a Roman general and statesman and a distinguished writer of Latin prose. He played a critical role in the gradual transformation of the Roman Republic into the Roman Empire....

's time the Atrebates

Atrebates

The Atrebates were a Belgic tribe of Gaul and Britain before the Roman conquests.- Name of the tribe :Cognate with Old Irish aittrebaid meaning 'inhabitant', Atrebates comes from proto-Celtic *ad-treb-a-t-es, 'inhabitants'. The Celtic root is treb- 'building', 'home' The Atrebates (singular...

 held land on both sides of the English Channel

English Channel

The English Channel , often referred to simply as the Channel, is an arm of the Atlantic Ocean that separates southern England from northern France, and joins the North Sea to the Atlantic. It is about long and varies in width from at its widest to in the Strait of Dover...

.

Linguistics

There is a hypothesis that the P-Celtic languages spoken in Gaul and in Great Britain

Great Britain

Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...

 were descended from a common ancestor. Gaul refers chiefly to France and the Low Countries. Gaulish is known from inscriptions and names from writings by Greek and Roman authors, as is Brythonic which is also known as Brittonic. There are a number of shared innovations that are not common between Brythonic and Goidelic as would be expected in the Insular Celtic hypothesis. However, these innovations could have been induced by contact in the Late Pre-Roman Iron Age or in the Roman period.

The shared innovations not in Goidelic are:

• Proto-Celtic kʷ > Gallo-Brittonic p (e.g. Gaulish mapos, Welsh mab ≠ Irish mac)
• Proto-Celtic mr and ml > Gallo-Brittonic br and bl (e.g. Gaulish broga, Welsh, Breton bro ≠ Old Irish mruig)
• Proto-Celtic wo, we > Gallo-Brittonic wa (e.g. Gaulish uassos, Welsh gwass ≠ Old Irish foss)
• Proto-Celtic ɡʷ > Gallo-Brittonic w
• Early loss of g between vowels in both Gaulish and Brythonic
• Proto-Celtic dj between vowels tended to give Gallo-Brittonic j
• First person singular verbs were suffxed in -mi in both languages
• Proto-Celtic *anman > Gallo-Brittonic anwan. (Gaulish anuana, Welsh enuein ≠ Irish ainm)