ATLANTA — Jamaica stunned the United States with two first-half goals, one off a blunder by goalkeeper Brad Guzan, and held on for a 2-1 victory in the Gold Cup semifinals Wednesday night.Darren Mattocks put the Reggae Boyz ahead with 31st-minute header off a throw-in. Giles Barnes followed five minutes later with a goal on an 18-yard free kick after Guzan was caught going outside the penalty area on a routine throw.Michael Bradley scored in the 48th minute for the Americans, but it wasn't enough to prevent a stunning setback in front of sold-out crowd at the Georgia Dome.