Gaínza made his La Liga debut on 13 October 1940 in a 0–1 away loss against Hércules CF, then proceeded to play a total of 19 seasons in the competition with the same team, amassing totals of 380 games and 119 goals. During his spell with the "Lions of San Mamés" he won nine major titles, including two national championships and seven Copa del Generalísimo trophies, plus a Biscay Championship and a Copa Eva Duarte (supercup). In May 1958, he received a testimonial match against English side West Bromwich Albion; coincidentally, the only goalscorer was Ronnie Allen who later became head coach of Athletic and organised further benefit matches between the clubs.