From an interview with John Frusciante in the April 2014 issue of Guitar Player:

"Was the Yamaha SG the only guitar you played on the last few records, or were there others?"

"I played a white Roland G-303 guitar along with a GR-300 synth on some pieces. For example, on "Cinch" I combined it with the SG parts on the arpeggio thing toward the beginning when the drums speed up, and I edited it back and forth with a Carvin nylon-string acoustic throughout the solo on "Breathiac," so they appear to be continuations of the same train of thought."

In the video upon here you can see a few frames of Fripp using this guitar starting from 1:26. Usually this guitar has been used by Robert with its original synth system (GR300), but occasionally it can also be seen plugged in and used through some roland synths along Robert Fripp himself during a few concerts in the 80s. He's been using this guitar from 1980 to 1984/85, maybe even 1993, before upgrading to his main instrument which still uses today.

This undated ad shows Andy endorsing the G-303 unit.

This photo shows Patrick "Pat" Bruce Metheny, legendary guitarist of acts such as the Pat Metheny Group, Jaco Pastorius, and Herbie Hancock playing his iconic Roland G-303 Guitar Synthesizer Controller 6 string. He is also noted using it in conjunction with the Roland GS-300 controller pedal.

Here's a rare live video of Adrian using a Roland G303 with Robert Fripp. During the live at Frejus in 1982, Adrian uses a Roland G303 for the song The Sheltering Sky (16:48). During King Crimson's period, Adrian rarely used a Roland G303 between 1982 and 1983, especially on the european tour

In this picture from Stop Making Sense, David can be seen playing the Roland G-303 Guitar Synthesizer

Guitarra usada por Charly a finales de Serú Girán y en su primer disco de solista

En el video se ve a Pedro ejecutar una Guitarra Roland G-303

Guitarist Chris Buono confirms on his official website that he owns a Roland G-303 Synthesizer Guitar, calling it "Super-fun."