GIS Mapping

i would like to know when can we have the gas mapping feature available in the qgroundcontrol?

Not quite sure what you are looking for. Can you provide some detail.

Just make an autonomous flight with QGroundcontrol and postprocess the data with one of the GIS software.

i wished to use GIS maps for mission planning in qgroundcontrol likewise i could do in the mission planner. with that i could have a better idea of the terrain while performing mission?
And also is it possible to add ban zone areas while mission planning?

Do you mean importing own raster files via WMS like in MissionPlanner ? And importing shape-files as polygons for survey missions ? I would like that too!

There is no way to add additional maps to QGC. GeoFence is supported if you are using the PX4 flight stack. It is not supported for ArduPilot flight stack. Support for importing KML to set polygons will be coming in the next version.