Gabrielle was having an affair with her husband Steven Deschain (the King of Gilead)'s top adviser at the time, Marten Broadcloak, but in truth Marten was actually a spy for John Farson, whom seeked to destroy Gilead and kill Steven and his band of Gunslingers. Eventually Roland found his mother with Marten and turned his back on her for good. When the time was there, Steven Deschain, tried to arrest Marten, but he fled. Eventually Gabrielle tried to better her ways, but she was eventually again lured in by Marten's deceitful ways. It was Marten that had instructed her to steal a special ornament (Maerlyn's Grapefruit) in the possession of her husband, and after that, kill her husband. It should have taken place during a celebration in witch Steven and Gabrielle would renounce their past mistakes and start anew. Roland however did not trust his mother and followed her when she left the party. It was then that Roland touched the ornament that Gabrielle was to steal. It transported Roland's mind to that of the Crimson King's domain. Roland tried to shoot the Crimson King, but then returned to his own body and world. He then saw that he had shot Gabrielle, his own mother, whom had been standing close to him. As Gabrielle lay slowly dying from her wounds, but not before saying that she loved Roland, her son. It seemed that at least Gabrielle was not resentful of her son Roland being the cause of her death, as her life had seemingly gotten out of her own control and the woman she once was, was no more.
The Dark Tower IV: Wizard and Glass
Roland, Eddie, Susannah, Jake, and Jake’s pet bumbler survive Blaine the Mono’s final crash, only to find themselves stranded in an alternate version of Topeka, Kansas, one that has been ravaged by the superflu virus. While following the deserted I-70 toward a distant glass palace, they hear the atonal squalling of a thinny, a place where the fabric of existence has almost entirely worn away. While camping near the edge of the thinny, Roland tells his ka-tet a story about another thinny, one that he encountered when he was little more than a boy. Over the course of one long magical night, Roland transports us to the Mid-World of long-ago and a seaside town called Hambry, where Roland fell in love with a girl named Susan Delgado, and where he and his old tet-mates Alain and Cuthbert battled the forces of John Farson, the harrier who—with a little help from a seeing sphere called Maerlyn’s Grapefruit—ignited Mid-World’s final war.