When a quarterback is called a "gunslinger," it relates to his mental toughness, confidence and competitiveness. From a tangible standpoint, the most notorious gunslingers have exceptionally strong arms.
Their arm strength is what gives them the confidence to take chances with the football. The most famous quarterback with a gunslinger mentality is probably Brett Favre, while Detroit Lions quarterback Matthew Stafford is likely the best active example.
Within the 2015 class, several quarterback recruits appear to play with a gunslinger mentality. A 5-star passer headed to Texas Tech isn't afraid to gamble, while Missouri has a pledge from a 4-star quarterback who is in love with his arm strength.