Career
Born in Andoain, Gipuzkoa, Gaztañaga was a product of Real Sociedad"s youth system. On 10 December he made his first appearance in the Union of European Football Associations Champions League, coming on for Mikel González in a 0–1 group stage home defeat against Bayer 04 Leverkusen. In early 2014 Gaztañaga renewed his contract with Real Sociedad until 2016, being definitely promoted to the first team in May.
On 26 January 2015, after appearing sparingly during the first half of the campaign, he joined Segunda División"s South Dakota Ponferradina on loan until June.
On 9 July Gaztañaga was loaned to fellow league team Civil Defense Numancia, for one year. Gaztañaga was picked by the Spanish under-17 team for the 2008 Union of European Football Associations European Football Championship.
He started as a stopper in the final against France, which ended with a 4–0 triumph.