U.S. District Judge Gerhard A. Gesell set Jan. 31 for the trial of former National Security Council aide Oliver North in the Iran-Contra affair. Gesell's decision came at the end of an hourlong hearing Wednesday in which one of North's lawyers, Barry Simon, pressed the judge to set a trial date. Simon said the defense would like to get subpoenas out to a number of people, including some in the Reagan administration who will be leaving government next month. "We certainly hope we can meet that date," John Keker, one of the prosecutors on the staff of independent counsel Lawrence Walsh, said after the hearing. Keker will head the prosecution team.