... Arvirargus made up his mind to give some special mark of distinction to the place where he had married [Genvissa]. He suggested to Claudius that the two of them should found there a city which should perpetuate in times to come the memory of so happy a marriage. Claudius agreed and ordered a town to be built which should be called Kaerglou or Gloucester. Down to our own day it retains its site on the bank of the Severn, between Wales and Loegria. Some, however, say that it took its name from Duke Gloius, whom Claudius fathered in that city and to whom he granted control of the duchy of the Welsh after Arvirargus. Once the city was built, and the whole island was at peace, Claudius returned to Rome, leaving the governorship of the islands of this province in the hands of Arvirargus. [IV.15]

No man was fiercer than Arvirargus in war and none more gentle in time of peace. Nobody was more agreeable to be with or more open-handed in distributing gifts. When he reached the end of his days he was buried in Gloucester, in the very temple which he had dedicated in honour of Claudius. [IV.16]

GLOUCESTER appears first in the Historia when it is founded by the British king Arvirargus, in cooperation with the Roman emperor Claudius, after the two have made peace and sealed the pact with the marriage of Claudius's daughter to the British ruler. There was a settlement in the city in Roman times.

Gloucester next appears as the death-place of Lucius, the king who converts to Christianity and converts his people as well:

In the end Lucius died in the town of Gloucester.... In the year 156 after the Incarnation of our Lord he was buried with all honour in the church of the Archdiocese. [V.1]

A certain Count of Gloucester, called Eldol, was among those present. The moment he saw this act of treachery, he seized a wooden stake which he had found by chance, and proceeded to defend himself with it. He broke the limbs of anyone within reach whom he could hit with this piece of wood, dispatching him forthwith to Hell. He shattered skulls, arms, shoulder-blades and even legs, causing the greatest possible terror, and before he left the spot he had killed seventy men with the stake which he held. He could not resist so vast a force for long, but got away and sought refuge in his own city. [VI.16]

Eldol, Count of Gloucester, is a central figure in the struggle with the Saxons. He is the sole survivor of the massacre on Mount Ambrius, and in book VIII, he sets himself against Hengist:

Meanwhile Eldol, Duke of Gloucester, went to the King and said: 'I would consider this one day a sufficient recompense for all the remaining days of my life, if only God would grant that I might fight hand-to-hand with Hengist.... As the various companies of troops moved forward in this direction and in that, by chance Eldol and Hengist were brought together, with no advantage to either of them, and they began to rain blows on each other.... When he set eyes on Gorlois, Eldol gained a new assurance. He seized hold of Hengist by the nasal of his metal helmet and by exerting all his strength dragged him in among his own men. [VIII.5-6]

Tatlock suggests that the heroic character of Eldol may have been intended as a compliment to Geoffrey's patron Robert of Gloucester (p. 47). The prominence of the town in the Historia is out of keeping with its significance in Geoffrey's day (p. 46); for example Eldol's brother Eldaldus is Bishop of Gloucester (he appears with that title in VIII.7, but appears elsewhere in books VI and VIII as well). Later, the Bishop of Gloucester becomes the Archbishop of LONDON [XI.3]. Tatlock points out that Gloucester in fact never had a bishop before the time of Henry VIII (p. 46), though the current Gloucester Cathedral stands on a very old monastic site.

Morvid, Earl of Gloucester, attends Arthur's plenary court [IX.12], and plays a role in the Battle of Saussy [X.12]

The city also appears in Merlin's prophecies:

An Owl will nest on the walls of Gloucester and in its nest will be hatched an Ass. [VII.4]

The City of Claudius will be proposed as a source of remedy and this city will put forward the Foster-daughter of the Scourger. She shall come bearing a saucer of medicine and in next to no time the island will be restored. [VII.4]