PCB design software typically does not concern itself with how to produce the PCB by milling - the normal output of a PCB design package are Gerber (RS-274X) and drill files. On the other hand, CNC controllers like bCNC or UGS typically don't directly interpret Gerber files, but expect GCode instead. There are of course exceptions - for instance I know there is some sort of plugin / tool for Eagle that produces gcode, although I'm not familiar with the details since I'm not using Eagle. I have no idea whether KiCAD has anything similar. Taking your PCB through other intermediate formats like dxf, svg or similar is not really a good idea.
That said, there are a number of tools specialized to bridge the gap, take in Gerber and produce gcode; Visolate, Line Grinder and FlatCAM are just the ones I know of. You can also give GCAM Special Edition (download) a try if you like, it's not specifically meant for Gerber-to-gcode but since it can do that too and I'm its maintainer I figured I'd mention it. All of these only create the appropriate gcode file, so you still need to use whatever sender you prefer (bCNC or other) to actually do the milling.