We still aren't sure how Guyan McHaffie (he has descendants pronouncing it Mehaffie) of 1750s Pennsylvania (in many records, sometimes as Mehaffie, and as Guian, Guion etc) relates to my banished McHaffie ancestor of Wigtownshire (banished Oct 1684). John McHaffie living in Arizona has done tremendous amount of work on Guyan and descendants and has wondered about the given name Guyan. The fact that your father's surname was Guyan, changed to Johnstone, makes me think there must be the surname Guyan in the British Isles. There was a Madame Guyonne (or similar) who was a couple of centuries ago, in France, a religious lady mentioned in more than one source as I recall. I've wondered if perhaps Guyan could be name of fugitive Huguenots of 16th century who went to Northern Ireland. Wish I could help more on Guyan family.