The problem was that it made no mention in the section on John Gibson of why the author thought that John Gibson was alive on that date.
While the author appears to have utilized local court, probate, and land records in the compilation, there are no specific citations to individual statements. My suspiscion was that there was some document that referenced John Gibson on that date.
The discussion of John Gibson (in Mehitable Calef Coppenhagen's John Gibson of Cambridge, Massachusetts and his descendants, 1634-1899, published in 1900) indicated that the compiler lost track of him in 1799.
Then I got to thinking.
What document could a researcher find without knowing where John Gibson lived that would indicate he was alive?
The answer was obvious: the estate settlement of his father.