I have never heard of this or had to do It?
I have used Orcad and Kicad from local PCB manuf to US and then offshore (China) and submitted just Gerber files.
No mirror required?
M.
You do not to mirror all the layer files in Gerber format. You just need to provide all the Gerber files and drill file to the PCB manufacturer. They know how to recognize them.
The standard (with our local PCB manufacturer's instruction PDF file) which accompanies the order form, states that ALL layers must appear in GERBER format, viewed TOP view, so the bottom layer will be seen as if viewed from the bottom. The CNC ( or other) software at their end will arrange it correctly.