So I have two questions here.
Is it believed, in general, that Geoffrey's work was mostly fictional and he knew it, or did he believe most of what he wrote?
How widely was the History of the Kings of Britain accepted as fact in its day? I know some, such as William of Newburgh, didn't accept its authenticity, but would the common man have mostly treated it as truth?
Thanks!
1 is a tricky question, Geoffrey draws on a number of sources, some considered very respectable at the time, which he may have regarded as truth. Other stories (the prophecy in Book One, Arthur killing a giant) read more like Geoffrey's take on the established genres of epic and romance and therefore unlikely to have been constructed with any sense of truth in mind. There are also a number of elements of Geoffrey's history that are innovations and surely their creator would have been aware of what he himself made up.
As far as whether it was accepted as fact. Michael Faletra says:
Although a few of Geoffrey's near-contemporaries expressed some skepticism regarding the truth-status of The History, his account was accepted in the main, and his version of the insular past remained authoritative both in Britain and on the continent for the next several hundred years.