The Americans battled back from a four-run deficit to within one in the eighth and had the bases loaded with two away.
Romero stayed in the game and retired the Americans in order in the ninth to clinch a spot for Puerto Rico in the semifinals.
Puerto Rico and the Dominican Republic will meet on Saturday in a game that will decide their semifinal opponents. The winner will meet the Netherlands in the semifinals on March 18 in San Francisco. The loser of Saturday's game will take on Japan on March 17.
The championship game will be played March 19.
Puerto Rico took a 1-0 lead into the sixth inning, then loaded the bases with two out and scored a run when Carlos Rivera drew a walk from Vinnie Pestano. That brought up Gonzalez, who came through with his double to left that delivered two runs for a 4-0 lead.