Three years ago, Edward Hirsch’s son Gabriel died at 22. Gabriel had gone to a party, where he was given a drug that sent him into cardiac arrest. For months afterward, his father,Edward Hirsch, was so overwhelmed by grief, he couldn’t function at work. But eventually, he turned to a familiar outlet. Hirsch is a poet, with eight published books to his name. He started writing down everything he could remember about his son. That exercise turned into “Gabriel: A poem,” a 78-page elegy published this month.