what will be connected to the X-10 switch? are there any neutrals in the existing three gang box? If this box [three gang] is fed off the existing knob n tube, then chances are there is no neutral in the box, just the hots and returns. the two coming off the three way are the travelers to the other remote three way. if you need a hot and neutral to the X-10, then running a piece of wire from the outlet you specified will do nicely. as far as gauge goes, whatever is on the gfi use that 12 or 14.