Hi, Nick. Given the time and location for this person, I would make sure that the Stephenson and McDuffie publications are using valid primary source documentation to show any ancestry for William Gatlin.
Taking a quick look at the McDuffie book (free at Family Search) it does appear well researched. I would use her citations that establish the relationship between William and his father John rather than just citing the book (but do give a link to the book at Family Search). I don't see McDuffie giving anyone earlier for William than his father John.
If you are researching these early Virginia profiles, please think about joining Southern Colonies project which has a special interest in these early Virginia profile.