I've never encountered the idea that it was satirical. I think the Historia is just too plain long to have been a satire. You've got to remember that most people thought it was entirely credible until the early modern era. If anything, more recent scholarship takes Geoffrey increasingly seriously. In Arthur and the Kings of Britain, Miles Russell argues that a lot of Geoffrey's stories are reflected in archaeology, and that, in many cases, he was providing distorted versions of ancient folklore. I'm sceptical myself, as are all the historians I've spoken to, but they consider the Historia to be an earnest forgery at the very least.

I'm intrigued as to which bits taken from Gildas and Bede you think seem like satire?