"Gunner" was a position held aboard Klingon Birds-of-Prey during the 23rd century, the officer holding which was responsible for weapon targeting and firing.
In 2285, Kruge, commander of a Klingon Bird-of-Prey, killed his gunner in a fit of anger due to the accidental destruction of the USS Grissom, a vessel which Kruge only intended to be disabled. (Star Trek III: The Search for Spock)
In 2287, General Korrd instituted Captain Spock as the temporary gunner aboard Klaa's Bird-of-Prey, so that Spock might destroy a malevolent alien entity which was attempting to kill Captain Kirk on the surface of the planet Sha Ka Ree. (Star Trek V: The Final Frontier)