Hi and welcome Leo,
I might be wrong, but I'm pretty sure the spokes are 12 gauge (2.6mm) but may have a reduced thread to enable smaller 13 gauge nipples to be used to fit the rims. I'm guessing that these spokes are specially manufactured to suit these hubmotors and are not likely to be available from any of the popular bike shops. A bike shop who specialises in electric bikes may be able to help, but I think you will probably need to get the correct spoke and nipple from GM (Unless one of their dealers has a damaged wheel to take one off).
Alan