The GR-300 had no MIDI (it had not been developed yet) and could initially only be played/controlled with a Roland G series guitar controller (Originally offered with the G-303, G-505 and G-808, later the G-202 and it was backward-compatible with the G-707 and the GK-1 pickup system, released a few years later.
This process required a hexaphonic pickup to be installed on the guitar, essentially sending 6 separate signals, one for each string on the guitar, to individual pitch-to-voltage conversion circuits in the synthesizer module.
The actual synthesizer module sat on the floor and had the rugged appearance of a large, bright blue guitar-type foot pedal (complete with carrying handles) and did not resemble an actual keyboard synthesizer at all.
Each pair of VCOs were harmonically locked to each string but could be tuned separately to play different pitches.
The GR-300 also featured a VCF with variable length sweep up and down, and an LFO to add pitch vibrato to the oscillators.