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.
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.