He was guitarist with The Brogues, then most notably with Quicksilver Messenger Service, where the complex interplay between himself and fellow-guitarist John Cipollina did much to define the unique sound of that San Francisco based band.

It was in 1965 when, as Gary Cole, he joined the Brogues, in Merced, California, and met future Quicksilver Messenger Service drummer Greg Elmore.

In late 1965 Duncan received a call from John Cipollina offering an audition for himself and fellow Brogues member Greg Elmore to join Quicksilver Messenger Service.

"[citation needed] By the beginning of 1970 Duncan rejoined Quicksilver Messenger Service along with singer/guitarist Dino Valenti which pushed the group toward a more folk rock sound.

[citation needed] By 1971 the original group had splintered with Cipollina, David Freiberg and Nicky Hopkins all leaving while Duncan, Elmore and Valenti continued to perform as Quicksilver Messenger Service until the end of the 1970s.