What I am looking for is to properly convert a gerber file to a dxf without causing any geometry break. Its better to have a function can merge all the objects in same layers before exportation.

You may "join" the individual gerber trace segments, and then export your DXF.  Here's a tutorial movie showing those steps:

Note:  During DXF or DWG export, you can also choose Fill Option "OUTLINE" which will merge and generate outline for any geometry boundaries.

Note: If you are using AutoCAD, we recommend exporting as a DWG instead of DXF.  DWG is the binary version of DXF and is more compact and compatible with AutoCAD.