Drivers for flashing gx15 are included in the gx15 upgrading tool:
http://sharp.hacked.in/gx15/the%20gx...0installer.zip
You must install this software, and when is istalled connect your phone WITHOUT battery to your usb cable (with charging funcion) and then windows will auto install drivers from the folder of gx15 upgrading tool.
Once drivers are installed open the program and flash the phone