Red Dead Redemption 2 is filled with main story missions, but that doesn't mean you should ignore the many sidequests that can be found throughout the game. They provide character development for Arthur Morgan as well as more information regarding the world.
The Gunslinger sidequest is one of the first sidequests players will stumble upon. It's available from Chapter 2 onward and can be started by heading into Keane's Saloon in Valentine. Note that this saloon is not the one found in the town's main street where several story missions take place, it's in the back streets.
Upon entering the saloon, Arthur can interact with a man who's sitting by the counter. He'll tell you about his desire to learn more about "some legendary gunslingers" who have wrecked havoc in the West. Arthur can agree to help him track them down and take pictures of them in exchange for a reward. You need to track down five legendary gunslingers and loot photographs from their bodies, or help them in exchange for the photos. The five targets are Billy Midnight, Emmet Granger, Black Belle, Flaco Hernandez and Slim Grant.
Defeating the gunslingers allows Arthur to obtain a variety of rewards, including some unique weapons. The weapons are dropped after duelling with the gunslingers. Just remember to loot and grab the weapons after the duel as leaving the area before picking them up makes the weapons disappear forever!
A fifth and final gunslinger can be hunted down only from Chapter 4 onward. Killing him and taking his photo completes the Gunslinger sidequest completely.
Billy Midnight can be found at the Rhodes Train Station. Once at the station, Arthur needs to board the next train and explore the compartments until Billy Midnight is found. Confronting him will lead to a chase sequence on the train, which will end on top of the train itself. There, Arthur and Billy engage in a duel. Upon victory, Arthur can pick up the gunslinger's unique weapon, the Midnight Pistol. Taking his photo ends the mission.
Black Belle can be found in Bluewater Marsh, near Rhodes. Upon reaching the homestead, the gunslinger will ask Arthur for help in defending her home against bounty hunters, who are coming from the front of the house and from the left. Several waves of bounty hunters have to be defeated in order to take a photo of Black Belle and complete the mission.
Emmet Granger is located at a homestead near Flatneck Station close to Flat Iron Lake. The gunslinger will ask Arthur to help in cleaning his hog pen. Once the task has been completed, Arthur will have to convince him to duel. Once Emmet has been killed, Arthur can take a photo of his body and complete the mission.
Flaco Hernandez can be found north of the Grizzlies West, close to a frozen lake. He is one of the most difficult gunslingers to take down, as Arthur may have to confront his whole gang as well. There are two different ways to handle the mission: threaten the gang to let Arthur pass, or simply fight all of them. Once the gang has been dealt with, Arthur will duel Flaco. Upon his defeat, it's possible to pick up Flaco's Revolver, a cigarette card and a treasure map, The Poisonous Trail Treasure Map is in his cabin.
Slim Grant is the final gunslinger and he is only available during Chapter 4.
At any time during the chapter, Arthur can head back to Keane's Saloon in Valentine to learn that the biographer he initially encountered is no longer in the area. After this, he has to go the Grand Korrigan in Saint-Denis, where more about Slim Grant is revealed.
Heading to the sheriff's office in Annesburg reveals the gunslinger's current location, which is near a bandit camp close to Kamassa River. Before being able to take a photo of Slim Grant, Arthur will have to rescue him from the bandits and lead him to the designated location on the map. Slim will challenge Arthur to a duel, and once defeated, it's possible to take a photo of him.
Slim Grant also drops the Calloway's Revolver, which shows the "Canis Canem Edit" phrase on both its sides, a reference to another Rockstar developed game, Bully. Once the photo has been taken, the Gunslinger quest is finally complete.