If you take a look at the gmapping page there're two parameters called maxUrange and maxRange. I quote: "If regions with no obstacles within the range of the sensor should appear as free space in the map, set maxUrange < maximum range of the real sensor <= maxRange".
So, if you want that region appears as free space, you have to set your maxUrange under the maximum range of the real sensor (which is the maxRangeparameter).