Have you read the GPL? It makes very little sense for hardware (it’s actually a pretty weird license full of all kinds of software specific things. “Linking” to “operating system components” for example doesn’t make them a derivative work, whatever all that means. There’s quite a lot of other stuff.)

The argument for using the GPL when creating copyleft software is that most copyleft software uses the GPL and you want everything to have a compatible license, that’s not really the case for hardware.