If you already have created a map then you don't have to use gmapping anymore. gmapping is for SLAM : Simultaneous Localization and Mapping so you just need to localize the robot in your map.
What exactly have you run to get this result ? I assume you've just launched map_serverto get your map and then gmapping ? Instead take a look at the file navigation.launch from your package, they launch the map_server node and then they use the package AMCL for the localization.