Gaínza retired at the end of the 1958–59 campaign at the age of 37, having appeared in 494 official matches which was a club best at the time of his retirement. Such was his length of service that in addition to being one of its youngest goalscorers he was also the oldest, having found the net for the final time against Sevilla FC in March 1959, aged 36 years and 298 days; that record stood for 58 years, until Aritz Aduriz surpassed it in December 2017.