"Gas Mask Man" is a side quest available in Dying Light.
Starting the Quest Edit
In order to continue, Kyle has to go talk to Shakur (who's out of town holed up next to a river) and he tells Kyle that someone was disguised as Shakur, and Shakur didn't really do it. However, Shakur doesn't reveal who the person disguised as Shakur was unless Crane gets Shakur's bag of valuables found at the bottom of the nearby river. Once Kyle throws over the bag to Shakur, he tells Kyle that he doesn't exactly know who did it, but it was one of the fishermen residing in the village.
Wrapping it Up Edit
Talking to a guard next to the village main gate reveals that only three people have keys to open the main gate. Crane is now tasked with searching Tom's hut, Musa's hut and Gursel's hut for a gas mask. The mask is located on a washbasin in Gursel's hut. After finding the gas mask, you have to go talk to Gursel. After talking to him, going over and talking to Musa completes the quest and you are rewarded with the exceptional Champion weapon upgrade.
Extra Notes Edit
When you first talk to Shakur, a new Safe Zone is opened up for you. That Safe Zone will become obsolete and will no longer be safe from enemies upon completion of the quest, with Shakur having abandoned the place and there are no other survivors.