It's the cable used to connect a guitar with a GK-2A pickup or any Roland compatible pickup to a GR synth.
Cable for use with MIDI guitar systems. It is compatible with Roland's GK/GR systems, V-Bass, GI-20, OC-20G, and other Roland systems that use the 13-pin cable for the hexaphonic pickups. The cable is also compatible with systems from other manufacturers that subscribe to the 13-pin standard, such as the G-50 from Yamaha, and pitch-to-MIDI systems from Blue Chip/Axon. Guitars which have the 13-pin cable connector built in include those from Brian Moore Custom Guitars and Godin.