I'm a descendant of Nathaniel, Benjamin's son, and I've been poring over any sort of records looking for the name of his wife. Can't find it.
Nathaniel has his own portion of the British Loyalists' website, however:
http://www.uelac.org/Loyalist-Info/extras/Gaffield-Nathaniel... Still, no wife's name. If anyone has information on that, it would be most helpful.