Licensing hardware under the GPL doesn't make any sense. The GPL was written to be applied to software written in compiled languages; it refers to its subject as "the Program", and places requirements on how you may "copy and redistribute" it, or make use of its "source code". It's not clear how this even applies to some types of computer applications (e.g, interpreted scripts), let alone to physical artifacts like a circuit board.
Most hardware these days has schematics and designs consisting of software. It's a fairly clear mapping between the source files and the compiled files (usually gerber files for circuit boards, EPS or DXF files for case designs, etc) that the terms of the GPL can still apply to.
