A former resident of the Georgica Association in Wainscott who is awaiting trial on charges of murdering his father in Manhattan was given a stern warning Monday by the judge presiding over his case.New York State Supreme Court Justice Melissa C. Jackson warned Thomas Gilbert Jr. to begin cooperating with the prosecution’s psychiatrist or risk losing his right to plead not guilty by reason of insanity. She also cautioned him that his refusal to cooperate might do him no good with the jury.Mr. Gilbert has been held without bail on Rikers Island since January 2015, when New York City police charged him with killing Thomas Gilbert Sr. in his parents’ Beekman Place apartment and then staging the scene to look like a suicide.On Feb. 15, Craig Ortner, the prosecutor in the case, told the court that Mr. Gilbert was refusing to speak to the state’s psychiatrist. Alex Spiro of Brafman & Associates, Mr. Gilbert’s lawyer, had previously indicated that he might present a defense arguing insanity at the time of the crime.Mr. Gilbert was scheduled to meet the psychiatrist yesterday. He is due back in court on April 18.