I did wonder what it was about naval gunners that made them more prone to falsifying records but looks like it is related to midshipmen working out celestial sights:
In the modern Navy falsifying reports, records and the like is often referred to as “gundecking.” The origin of the term is somewhat obscure, but at the risk of gundecking, here are two plausible explanations for its modern usage.
The deck below the upper deck on British sailing ships-of-war was called the gundeck although it carried no guns. This false deck may have been constructed to deceive enemies as to the amount of armament carried, thus the gundeck was a falsification.
A more plausible explanation may stem from shortcuts taken by early midshipmen when doing their navigation lessons. Each mid was supposed to take sun lines at noon and star sights at night and then go below to the gundeck, work out their calculations and show them to the navigator.
Certain of these young men, however, had a special formula for getting the correct answers. They would note the noon or last position on the quarterdeck traverse board and determine the approximate current position by dead reckoning plotting. Armed with this information, they proceeded to the gundeck to “gundeck” their navigation homework by simply working backwards from the dead reckoning position.
First time I recall hearing it was regarding finishing up a form with missing info: Something along the lines of:
E- 6:" I need that form right now."
E-4 “I haven’t finished filling it out yet:”
E-6 “Just gundeck something in there, I need it now”