Gregory Banks, a former gang member who confessed to a 1983 murder and served seven years before the conviction was dismissed, testified yesterday at Jon Burge's perjury trial that he was beaten and suffocated during interrogations by Chicago police detectives under Burge's command. Banks said that when he denied any involvement in the murder, a sergeant "pulled a nickel-plated handgun and put it in my mouth." He alleges officers later knocked him across the chest with a flashlight and wrapped a plastic bag around his head, preventing him from breathing. Federal prosecutors are expected to close their case next week.