The shooting happened at about 1 a.m. at a home in the 2200 block of San Remo Way, where the victim, Giovanni Bey, was renting a room, police said.
Investigators believe Bey had been trying to join a gang to which the suspected shooter, 29-year-old Isaac McCuan, belonged, police said.
A member of the gang had allegedly told Bey to assault someone as a way to show his alliance to the gang, but there was a misunderstanding in how the order was given and Bey refused to do it, police said.
McCuan allegedly came over to Bey's house and confronted him about it. During the confrontation, McCuan allegedly shot Bey, police said.
Bey died from a gunshot wound to his chest, according to the coroner's office.
Shortly after the shooting, police identified McCuan as the alleged shooter and obtained a no-bail warrant for his arrest.
According to police, McCuan has an extensive criminal history that includes assault, gang-related crimes, property crimes and firearms offenses.
McCuan is listed as a transient but has ties to several east Contra Costa County cities, police said.
Investigators have searched several locations, including one outside Contra Costa County.
Anyone with information his whereabouts is asked to call the Pittsburg Police Department tip line at (925) 427-7369.