A Queens man threatened his girlfriend with a gun before heading back to the apartment they shared, lighting a mattress on fire and killing himself, police sources said. Police were called to the apartment on 79th St. near Astoria Blvd. North in Astoria around 1 p.m., just after the 41-year-old man brought a gun to a hospital where his girlfriend worked and threatened to kill her, cops and sources said. Firefighters responding to the Barclay Gardens Condominium Development, found the third-floor apartment on fire, the FDNY said. “He came home and turned the gas on and killed himself because he knew the cops were looking for him,” said Maria Germini, who manages the complex. “That’s when we got the call, because of the gas. We didn’t want him to blow up the whole complex.” The man was discovered with a self-inflicted gunshot wound, police said. Firefighters arriving on the scene found that a mattress had been set on fire and quickly put out the blaze. Sixty smoke eaters from 12 different units were able to get the fire under control by just after 3 p.m., officials said. The man and his girlfriend rented a condo at the development, according to Germini. His name was not immediately released. Fire marshals were continuing to investigate the blaze early Wednesday evening.