who are the lawmen for such fighters?
Pat Garrett and Wyatt Earp to name two of them
Gunfighter
 Others, like Billy the Kid, were notorious bandits, and still others were lawmen like Pat Garrett and Wyatt Earp. 
who has the fastest hands in the west UNANSWERABLE what is the origin behind the term gunfighter? The term "gun slinger" was used in the Western film "Drag Harlan" in 1920 and was soon adopted by other Western writers. Gunslinger and Gunfighter are words used in a similar sense. are there any gunslingers in the modern world? Yes, but as participants in organized competitive sports like action shooting events, harking back to the Wild West. what are the usage of such fighters? Being hired for contract killings or at a ranch embroiled in a range war where they would earn "fighting wages". Some were bandits or outlaws, while others were lawmen.
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Gunfighter [SEP] Usage
Often, the term has been applied to men who would hire out for contract killings or at a ranch embroiled in a range war where they would earn "fighting wages". Others, like Billy the Kid, were notorious bandits, and still others were lawmen like Pat Garrett and Wyatt Earp. A gunfighter could be an outlaw—a robber or murderer who took advantage of the wilderness of the frontier to hide from genteel society and to make periodic raids on it. The gunfighter could also be an agent of the state, archetypically a lone avenger, but more often a sheriff, whose duty was to face the outlaw and bring him to justice or to personally administer it.