Born in Bilbao, Biscay, Basque Country, Larrazabal started his career with Lauro Ikastola CF, the team affiliated to his school in the town of Loiu. In 2015, he had a successful trial with Bilbao-based youth clubDanok Bat CF; it took until the latter part of his season there to establish himself in the team in the 'Juvenil' league, but by its end he had drawn interest from senior clubs. On 3 June 2016, after finishing his formation, Larrazabal signed for newly-promoted Segunda División B side Zamudio SD. He made his senior debut on 27 August 2016, playing the entire second half in a 1–3 home loss to Bilbao Athletic, and scored his first goal on 10 December by netting the last in a 3–1 home defeat of CD Toledo. He finished the campaign with three goals in 29 appearances, although Zamudio finished bottom of the table and were relegated. On 5 April 2017, Larrazabal agreed to a two-year contract with Athletic Bilbao, effective as of 1 July; upon arriving, he was assigned to the B-team. He spent two seasons with Bilbao Athletic in the third tier, playing regularly for the team and scoring nine goals in 2018–19. On 14 May 2019, Larrazabal agreed to a new two-year deal with the Lions, being definitely promoted to the main squad. He made his first team – and La Liga – debut on 24 August, coming on as a substitute for Óscar de Marcos in a 1–1 draw away to Getafe CF.
Larrazabal's father is Aitor Larrazábal, who made over 400 appearances for Athletic Bilbao as a defender between 1990 and 2004 and later became a manager, and whose goal celebrations included producing a pacifier from his shorts to represent his baby sons. They met as opponents when Gaizka was playing for Zamudio and Aitor in charge of SD Amorebieta, and again when Bilbao Athletic faced Barakaldo CF. In 2002, Aitor lined up in the same Athletic Bilbao team as Aritz Aduriz, who was also on the field for Gaizka's debut for the club nearly 17 years later. The Larrazabals were the first father and son to play for Athletic since goalkeepers Carmelo and Andoni Cedrún, and the first to both feature for the Basque Country team since 'Periko' and Xabi Alonso.
