"Gassed Up" is a side quest available in Dying Light.
Gas has turned off all around the city and your job is to turn it back on. A survivor named Jeff knows the system but he has moved out of the tower. He now lives in a reinforced building called Fort Jefferson. When you go to him he says he turned off the gas to do things to his fort. He says that you have to turn the 3 valves to get the gas back on. After doing that he will send you to the central pumps to turn off some pumps. You go back to his fortress and he activates his defense system. His fort blows up and that completes the mission.
Don't forget to check his ruins for the free double barrel shotgun.