With an announcement by legislators that the proposed $16 billion Gateway Tunnel under the Hudson River between New Jersey and New York has received a historic $6.8 billion Federal Transit Administration grant, project officials said now, the hard part begins. “Raising the money necessary is extremely (hard), building it is just as hard. We now have to build it,” said Anthony R. Coscia, Amtrak board chairman and Gateway Development Corporation vice president, in an interview. Gateway will build two new Hudson River Rail Tunnels and rehabilitate the 114-year-old original tunnels built by the Pe…