Gareth King's dictionary has a fair amount of good information about gwybod and nabod and their variations. He says that gwybod is (to know) a "fact, etc." and nabod is (to know) a "person, place, etc." He also notes that both:
As an example he gives: I new that = o'n i'n gwybod hynny
There's also a box on the Welsh-to-English side of the dictionary for Gwybod - 'to know' that has a lot of grammatical information about its use (takes up nearly a whole page!).
As an aside, I looked up adnabod for which he gives recognize but there is also a note that