Early Roland Guitar Synthesizers
Roland pioneered the nascent "guitar synthesizer" field in 1977, with the introduction of the GR-500/GS-500-- an ungainly guitar married to a large analog synthesizer unit via a cable the size of a garden hose.
Over the next several years, before the MIDI specification was adopted, Roland offered a series of guitars with built-in hex pickups, for use with their GR-300 analog synthesizer and GR-100 unit, which applied analog synthesizer-style sound shaping to sounds from the hex-fuzz generator built into the guitar. These instruments-- the G-202, G-303, G-505, and G-808-- included built-in controls for several synthesizer functions, and used an impressive 24-conductor cable with massive connectors to interface with the synthesizer or hex fuzz unit.
With the advent of MIDI, Roland introduced two new systems. The G-707 guitar, an angular instrument with a stablizer bar joining headstock and body, was offered with the GR-700 analog synthesizer. The GR-700, a huge pedalboard, included enough memory for 64 different patches-- and a MIDI Out jack. However, the MIDI signal was confined to MIDI Channel 1 only, and no pitch-bend data was transmitted-- two very severe limitations upon potential expressiveness. The G-707 also lacked the hex-fuzz generator featured in the earlier G-*0*-series guitars.
Roland also made it possible for guitarists to MIDIfy their own instruments for the first time, with the introduction of the rack-mounted GM-70 guitar-to-MIDI converter and its companion GK-1 hex pickup. The GM-70 allowed each string to control a different MIDI channel, opening the door to pitchbending as well as much more complex synthesizer orchestrations. This unit also made all of Roland's pre-MIDI G-*0* guitars into potential MIDI instruments, since the cable and connector configuration was the same.
At least half a dozen manufacturers made guitars with implanted Roland electronics-- not only the hex pickup, but controls for synthesizer modulation and filter characteristics. These included Gibson Les Paul and Explorer models, the Steinberger GL2-GR and GL2T-GR, the Hamer A7 Phantom, the Zion Turbo Synth, the M V Pedulla MVP-S, and the Modulus Graphite Blackknife Special Synth Controller. None of these instruments was a mass-market item.
Roland joined the digital synthesizer explosion with the introduction of the GR-50, a rack-mounted guitar-to-MIDI converter which included a built-in digital synthesizer module. With the GR-50, Roland introduced a new hex pickup, the GK-2-- and a new cable, with 13 conductors and a smaller DIN connector. This is the cable still in use with today's thinner GK-2A pickup. Thus, the GR-50 is fully compatible with the current Roland and Roland-compatible gear.
The Ibanez MIDI guitar system, which included the unorthodox IMG2010 guitar and the rack-mounted MC1 MIDI Converter/Programmer, used the same connector/cable configuration as the early (24-conductor cable) Roland equipment.
It's as rare as hen's teeth now, but for a time Roland made the BC-13 Bus Converter, to convert the signal from a 24-pin Roland (or Ibanez) guitar to the current 13-pin format. With one of these, your G-303 or G-707 could play a GR-30.
There's a lot more information on the early Roland gear in the Roland Equipment Specs Digest.
The information on this page reflects very little personal experience on my part; I gleaned it (over a period of years) from dead tree publications and online sources, in an effort to dispel my own confusion. Any errors are purely mine, and if you find any, please email me.