@cine There is no such things as "GPL", there is the GPLv1, GPLv2 and GPLv3.

Those are copyright licenses, so they cannot be applied to a manufacturing process, or even the list of manufacturing steps, as recipes don't fall under copyright (you could add something creative to a recipe to make it fall under copyright, but someone could just go and delete anything creative to leave a non-creative sequence of steps to follow).

The only thing that can really restrict a manufacturing process is a patent, so you can just leave it unpatented and you won't need to bother coming up with a free patent license.