Hardware isn't subject to copyright. There are places in China that can reverse engineer boards from hardware back to design files, so GPLing board designs is pretty pointless.

There are patents, but they don't protect designs or individual physical items, just processes.

There is IC mask protection, but that doesn't protect modified versions of the IC masks (if I understand it correctly):

https://en.wikipedia.org/wiki/Integrated_circuit_layout_desi...