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I am a medievalist specializing in the 12th Century.

There's a lot to unpack with Arthurian legend because the line becomes very blurred. Geoffrey himself opens the Historia Regum Britanniae with:

While my mind was often pondering many things in many ways, my thoughts turned to the history of the kings of Britain, and I was surprised that, among the references to them in the fine works of Gildas [De Excidio et Conquestu Britanniae] and Bede [Historia Ecclesiastica Gentis Anglorum], I had found nothing concerning the kings who lived here before Christ's Incarnation, and nothing about Arthur and the many others who succeeded after it, even though their deeds were worthy of eternal praise and are proclaimed by many people as if they had been entertainingly and memorably written down. I frequently thought the matter over in this way until Walter archdeacon of Oxford, a man skilled in the rhetorical arts and in foreign histories, brought me a very old book in the British tongue, which set out in excellent style a continuous narrative of all their deeds...

It is true that neither Bede nor Gildas mention Arthur by name, though they do name many of the other figures in Geoffrey's narrative, such as Hengist and Horsa, and Vortigern. The Introduction to the text edited and translated by Michael D. Reeve and Neil Wright, also point out many similarities between Geoffrey's work and Nennius' Historia Brittonum, which he does not specifically cite. However, the identity of the "very old book" is never given specifically. That Geoffrey seems to be readily familiar with Arthurian stories, and that he says "as if they were entertainingly ad memorably written down" indicates that not only had they not previously been recorded in a format Geoffrey had seen (the Annales Cambriae don't seem to be referenced in connection with Geoffrey of Monmouth, though they also mention Arthur by name), but that stories of King Arthur had already passed into oral tradition by the time Geoffrey was writing. It is important to remember that we only have a fraction of Medieval writings, and there is a great deal that has been lost.

We don't have any way of knowing, really, if Geoffrey thought he was writing down facts, or if he was just recording a popular legend. Monmouth is now located within Wales, but during the twelfth century, the actual border would have been constantly changing as marcher lords and Welsh princes quarreled. It is very likely that Geoffrey would have been familiar with colloquial stories, and he may have had Welsh ancestry, though we can't be sure as he provides little in the way of autobiography. In fact, he may have also been of Breton origin, though we can never know for certain.

Geoffrey also told us who his intended audience was:

Though I have never gathered showy words from the gardens of others, I was persuaded by his request to translate the book into Latin in a rustic style, reliant on my own reed pipe; had I larded my pages with bombastic terms, I would tire my readers with the need to linger over understanding my words rather than following my narrative.

Geoffrey did not mean for his History to be read only by other clerics and scholars, but by secular aristocrats, whose knowledge of Latin would not have been as carefully cultivated. This is important to note when considering Geoffrey's style and bias throughout the text. He is not only writing to pass on information, but to convey a message to his audience about how they ought to behave, and what ideals they should espouse. Geoffrey dedicated his History to two leading nobles: Robert of Gloucester, bastard son of King Henry I and leading supporter of Empress Matilda during the Anarchy, and Waleran of Meulan, a member of the powerful Beaumont faction who largely supported Stephen, though it must be noted that the dedication to Waleran does not appear in the text as consistently.

We are in luck that we have several medieval scholars who were writing at the same time as Geoffrey: Orderic Vitalis, Henry of Huntingdon, and William of Malmesbury. Orderic had undertaken initially to write a history of his monastic house, but his work eventually expanded until he had written effectively a history since the time of Christ, which also included long sections about England, Normandy, and Anjou, and the First Crusade. Orderic intended to write a serious history, in line with Bede; he even titled it as the "Ecclesiastical History", presumably in homage to Bede. Henry of Huntingdon, like Geoffrey, was writing a history he intended to be read by secular people, and often wrote with the intention of leaving behind moral lessons. William of Malmesbury was another serious scholar, and probably the most sophisticated of his time. We can surmise, with reasonable certainty, that William of Malmesbury was a source for both Orderic and Henry of Huntingdon, and we know that Orderic was sent to visit William for research purposes.

To start with Orderic's views on Geoffrey, we know that he certainly read Geoffrey's writings (though he may not have seen a completed version of the Historia Regum Britanniae before his own death). Orderic cited instead the Prophecies of Merlin, which is often included with the Historia. Orderic does not reference Geoffrey directly, but states:

If anyone withes to know more details about this and other events concerning the Britons, he may study the books of Gildas, the British, and Bede, the English, historian, in which the reader will find a brilliant account of Vortimer and his brothers and of the brave Arthur, who fought twelve battles against the English.

Marjorie Chibnall, the editor and translator of Orderic, notes that Orderic incorrectly attributes his knowledge to Gildas instead of Nennius (a similar omission to Geoffrey), and that this would be the first extant work in which Geoffrey of Monmouth would have been referenced. Orderic then goes on to include a section of the Prophecies of Merlin, and then attempts to connect them with current events. This indicates that Orderic at least takes these stories as truth.