According to Stephen King's Dark Tower series, that would be Roland Deschain of Gilead, affectionately known as "old long tall and ugly", by his friend Eddie Dean of New York.
According to this node, a gunslinger was a freelance killer in the old American West. How such a low life became the noble knight of Stephen King's tale is just a continuation of the process by which the dirty, often illiterate laborer known as the cowboy became a dashing character in American history.
Roland is the last of a breed of men who acted as law enforcers in a world that looks a bit like the one in A Canticle for Leibowitz. However, they were not just cops, they were (sort of ) knights as well, or such was the impression I got from the books. Experts in all sorts of weaponry, their most distinctive weapons were big revolvers. I have never seen, much less handled a revolver but I imagine the famous Colts or Smith & Wessons of cowboy movies would fit the description perfectly.
Roland is the last because his world has changed; in the language of the book, it has moved on. His society which produced the gunslingers had been destroyed, and no more were being trained. During his journey, he trained 3 of his companions, Eddie Dean, Odetta Holmes and Jake Chambers to be gunslingers as well. I don't know if they would have qualified as proper ones had his world had not moved on. I am certain Odetta Holmes (who is a woman) would not have because there is no mention of female slingers in the story. 2 of these 3 companions die before the end of the quest and the 3rd decides not to continue traveling with Roland. That is how he became the last gunslinger.
The Dark Tower series is one of the best stories I have read. I occasionally reread all the books.