A Long Island architect accused in a string of long-unsolved slayings known as the Gilgo Beach killings is expected to plead guilty, closing a case that bedeviled investigators, agonized victims’ relatives and tantalized a true-crime obsessed public for years. Rex Heuermann is charged with murdering seven women, many of them sex workers. A guilty plea would put him in prison for the rest of his life. Last year, a judge rejected his bid to exclude DNA evidence obtained through advanced techniques that prosecutors say proves he’s the killer. He previously pleaded not guilty. A crowd gathered Wednesday waiting to get into the court hearing. Reporters swarmed Heuermann’s ex-wife, Asa Ellerup, and their daughter as they walked in.