Hello Richard, As a slang term it refers to one who performs at the pinnacle of professional ability , rank or prestige ; one who is the best at what one does,regardless from the profession. In a movie like 'Top gun' of course it refers to the best Air force pilot.

Top Gun. Top in this context means "the most best" Gun can mean "soldier" (soldiers are "hired guns") Top Gun therefore can mean the best soldier (in English this is typically in reference to Airforce pilots only due to the movie "Top Gun" (1980 something)

in my own opinion top gun means a good quality of gun. cowboy...