Gavin started to work professionally in 1979. He worked as a freelance session drummer on records and tours for the following artists: Incognito, Lisa Stansfield, Lewis Taylor, Artful Dodger, Paul Young, Iggy Pop, Level 42, Porcupine Tree, OSI, King Crimson, Shooter, Dizrhythmia, The Pineapple Thief, The Kings Of Oblivion, Sam Brown, Tom Robinson, Go West, Black, Gail Ann Dorsey, B J Cole, Dave Stewart and Barbara Gaskin, Nathan East, Yasuaki Shimizu, Camouflage, Kevin Ayers, Claudio Baglioni, Franco Battiato, Chizuko Yoshihiro, Renaissance, Mick Karn, Eros Ramazzotti, Nick Johnston, Randy Goodrum, and Fates Warning.
In 2002 he joined Porcupine Tree and has played on the band's albums released since that time: In Absentia, Deadwing, Fear of a Blank Planet, The Incident, and Closure/Continuation, with each receiving critical acclaim and raising the band's status to one of the most influential modern progressive rock bands. He remained a permanent member of the band until an indefinite hiatus was announced in 2010, and then again in 2021 when the band announced a return to playing together with the release of the album Closure/Continuation.
In 2007 Harrison began a long term collaboration with singer/extended range bass player 05Ric, which led to the release of three CDs, Drop (2007), Circles (2009) and The Man Who Sold Himself (2012).
In 2008, Harrison joined King Crimson as part of a dual-drummer line-up with Pat Mastelotto. He played a number of shows in the United States in August with the band. He also recorded drums on Steven Wilson's debut solo album, Insurgentes.
On 23 August 2011, he was a featured performer on the 'Late Show with David Letterman' as part of their second "Drum Solo Week", along with such players as Sheila E, Stewart Copeland, Neil Peart, and Dennis Chambers.
As of September 2014, Harrison plays live in the new formation of King Crimson, as one of the three drummers.
In 2016, he joined The Pineapple Thief as a session drummer for the band's 11th studio album Your Wilderness, which received widespread critical acclaim and an overwhelmingly positive fan reception. He joined the band on the tour following the album in January 2017. In August 2018, just prior to the release of Dissolution, the band announced that Harrison had officially joined the band as a full member. He has since been actively involved and credited in the band's songwriting and album production together with the founder Bruce Soord.
