Gaynor joined Simple Minds in 1982 as a session drummer for the New Gold Dream album (as a recommendation by the record producer, Pete Walsh) then later joined the band permanently for the New Gold Dream tour, as a replacement for Mike Ogletree. Except for a brief stint away from the band after the Real Life tour in the early 1990s, he has been the band's drummer to the present day.
Gaynor began drumming at age 11 and had his first professional engagement at age 14.
In addition to drumming, he is active as a songwriter and a record producer. Currently, he has also been working on a side project as part of a duo (along with Stephane Deriau-reine) called The Fusion Project. The Fusion Project's first album was released in April 2006 and is described as "a mix between progressive rock and jazz-fusion."