how to convert HFSS file to gerber file?
The issue is probably that you have an antenna file that is in a 3D format. You (probably) need to convert to 2D like .dxf that is more easily converted to Gerber files. I am assuming that you are trying to make some sort of printed circuit antenna.
There are other solutions possible and you might find free software somewhere. Even with paid software, translations from one to another can be problematic.
i know there is option of exporting our design in hfss to .dxf, but my design comprises of conical horn like structure so how to convert this to 2D structure of .dxf,?
What 2D export do you expect for a 3D object? You would need to define a 2D cutting plane where the 2D view is defined.
Why would you want a Gerber to produce a conical horn? Maybe I am missing something in your question. If you have a 3D object like a horn I would expect that one of the 3D formats would be more appropriate for a build. Examples are :*.sld, *.sat, *.igs and the like.
In passing, if you export a *.dxf from HFSS what you get is a cross section of whatever passes through the xy plane. You can position your solid model and export 2D cross sections all day to try to describe the 3D shape.
An ad just appeared in my email from w*w.pentalogix.c#m. (I am sure you can figure out the actual URL) They have some sort of Gerber tool called PCB Analyzer for S295 that may address your need.
You might want to take a look at it to see if it is useful. I have no knowledge of the package nor am I associated with Pentalogix.
Good Hunting.