Gaynor began his career playing juvenile football in Ardee with Square United and Ardee Celtic before moving on to
Belvedere . Gaynor played for the Dundalk Schoolboys League side. He opted to begin his career with
Millwall. Gaynor later joined
Sutton United on loan in 2007 but was released at the end of the 2007–08 season, subsequently joining
Cobh Ramblers. Gaynor then signed for
Sporting Fingal in December 2008 for the season, but joined
Drogheda United on loan in July 2009. On 9 February 2010 he signed for
Dundalk. In December 2011, Gaynor signed for
Sligo Rovers. Despite starting out on the wing he eventually settled into the left back position due to an injury to regular left back Iarfhlaith Davoren. He was an important part of the defence that secured a first league title in 35 years for Rovers while also providing several assists. He made a good start to the 2013 season again in the left-back position and scored his first league goal for the club in the home win over Derry City. An injury towards the end of the year restricted his appearances but he added an FAI Cup winners' medal to his collection coming on as a sub in the 3–2 win against Drogheda in the final. ==International==