Gerba gives Canada second Gold Cup success
Canada lead Group A while Costa Rica, who beat Jamaica 1-0 in Tuesday's other game, are level with El Salvador on three points. Jamaica are bottom of the group after two defeats.
Gerba turned in a shot from Patrice Bernier after El Salvador failed to clear and the Canadians are now all but certain to make the quarter-final stage.
Celso Borges's 64th minute goal was enough to keep Costa Rica's hopes of progress alive and condemn Jamaica to another defeat.
Borges met a Pablo Herrera cross with a superb volley to beat Jamaica keeper Donovan Ricketts.
Jamaica were forced to play with 10 men from the 35th minute after Rudolph Austin was sent off following a clash with Borges.