Here GW means man-days. Hence, if four of the six slave carpenters GW had in 1765 worked a 5½-day workweek they would have put in 22 man-days for that week.
“[Diary entry: 22 September 1765],” Founders Online, National Archives, https://founders.archives.gov/documents/Washington/01-01-02-0011-0007-0004. [Original source: The Diaries of George Washington, vol. 1, 11 March 1748 – 13 November 1765, ed. Donald Jackson. Charlottesville: University Press of Virginia, 1976, p. 341.]