Gaynor played his club hurling with his local Kilruane MacDonagh's club. He enjoyed some success at underage levels before later winning three consecutive senior county championship titles in 1977, 1978 and 1979.
Minor and under-21
By the early 1960s Gaynor had made an impact as a member of the Tipperary minor hurling team. He tasted his first success in 1962 when he won a Munster minor medal. He later joined the Tipp under-21 panel with whom he won both Munster and All-Ireland medals in 1964 in the inaugural year of the competition. Gaynor won a second Munster under-21 medal in 1965.
Senior
Gaynor's skill at underage levels brought him to the attention of the senior selectors who elevated him onto the senior county team in 1964. He went on to win his first National Hurling League medal at the start of 1965, before Tipp demolished all opposition in the provincial championship giving him his first Munster title. Wexford were Tipp's opponents in the All-Ireland final, however, the game failed to live up to the two classic games between the two sides in 1960 and 1962. Victory went to Tipp on that occasion with Gaynor collecting his first All-Ireland medal. Tipp lost their provincial and All-Ireland crowns in spectacular fashion at the first hurdle in 1966 when a young Limerick team took them by surprise. The team bounced back in 1967 with Gaynor collecting a second Munster title. By this stage, however, Tipp's pool of players was ageing and the county's hurling fortunes were in decline. Kilkenny provided the opposition in the subsequent All-Ireland final and provided more than a match for Tipperary. Goals from Paddy Moran, Martin Brennan and Tom Walshlaid to rest a bogey that Tipperary had over Kilkenny since 1922. Tipp crushed Cork in the provincial final again in 1968, giving Gaynor his third Munster title. For the fourth time that decade Tipperary faced Wexford in the championship decider, however, the Munster champions were now in terminal decline and surrendered an eight-point lead at half-time giving Wexford a 5–8 to 3–12 victory. In 1971 Tipp defeated Limerick in the provincial decider, giving Gaynor a fourth Munster winners' medal. Kilkenny provided the opposition against Tipperary in that year's All-Ireland final. In the All-Ireland final to be broadcast in colour by Telfis Éireann, Tipp won on a score line of 5–17 to 5–14. It was Gaynor's second All-Ireland medal. Gaynor retired from inter-county hurling in 1973.
In the early 1990s Gaynor served as manager of the Clare senior hurling team. He had some success by steering the county to two consecutive Munster final appearances in 1993 and 1994, however, these games ended in huge defeats for Clare. He was replaced in 1995 by Ger Loughnane. In 1997 Gaynor became manager of the Tipperary senior hurling team. Once again his tenure in charge was mixed, as Tipp were defeated in both the Munster and All-Ireland finals in the first year of the "back-door" system.
