https://go.gale.com/ps/i.do?id=GALE%7CA15380511&sid=googleScholar&v=2.1&it=r&linkaccess=abs&issn=00945404&p=AONE&sw=w&userGroupName=cens39029

Because of the likelihood that Gray is a misspelling, it is most likely the case that he's referring here to Richard Grey)'s method from the book Memoria Technica, or, a New Method of Artificial Memory (1730).

Could they have known each other personally? Might be worth checking his massive correspondence.