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, Fates Warning.

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.

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 Stewart Copeland, Neil Peart, and Dennis Chambers.

Harrison won the Modern Drummer readers’ poll for “best progressive drummer of the year” consecutively from 2007–2010 and again in 2016 and 2019. He won “Best Prog Drummer” in DRUM USA magazine 2011. Prog voted him best drummer in 2011, 2012, 2018 and 2019. He is the featured cover story on Modern Drummer January 2009 and February 2015. Rolling Stone polls rate him as the third best drummer in the past 25 years. In 2014, Modern Drummer magazine placed Harrison in the “Top 50 Greatest Drummers of All Time”.

In January 2017 he was guest drummer with The Pineapple Thief for European tour in support of their album Your Wilderness (in which he also played all the drums). ”We’re so excited that we have made it happen. It’s a real once in a lifetime event”, said their leader Bruce Soord.

In August 2018, just prior to the release of The Pineapple Thief’s 12th studio album Dissolution, the band announced that Harrison officially joined the band as a full member.