I have the information you are looking for. I am just about to publish a book (See Lee Perryman's review above) that has John Gambrell's ancestry, his father and the others before him in it. The John Gambrell who had a "brother, b. ca. 1774", was not John Newton Gambrell. The latter is his grandson. The John Gambrell you refer to (1755-1834) is named just so, not Newton.