All I know about him is what Williams wrote in his Folk Songs of the Upper Thames (1923, p. 15):

"Four such champions I myself knew and will mention their names: John Pillinger, of Lechlade; David Sawyer, of Ogbourne; William Warren, of South Marston; and Gabriel Zillard, of Hannington. Of Zillard it is said that he would unbutton his shirt-collar at six in the morning and sing for twelve or even eighteen hours, if necessary, with the perspiration streaming down his cheeks. From what I have heard of his songs, they were very quaint and comical; unfortunately, by reason of his infirmity, I was unable to obtain possession of any of them."

~Masato