I recently wanted to panelize a couple designs into a gerber set.
The answer is easy, but there are some gotchas. I did a writeup here: https://kicad.mmccoo.com/2017/12/28/panelizing-kicad-gerbers/
Here’s the short answer:
This hackaday tutorial is almost enough.
you need to rename the gm1 file generated by pcbnew’s plot to gko
you need to rename the drl file to txt
you need to use the “use protel extensions” option.
hope it helps