Giannakopoulos began his career in the youth team of
Asteras Tripolis, after which he joined the youth team of
Blackburn Rovers, and after a year at Blackburn Rovers, he joined the youth team of Italian club
Udinese Calcio. Eventually, Giannakopoulos was released and returned to his country to join
Panionios. On 27 August 2012, he made his league debut against
Aris. Giannakopoulos has been the first-choice keeper for the club since the early part of 2013. There was a brief period when
Kostas Peristeridis displaced him from top spot again after young Giannakapoulos oversaw a run of 3 straight defeats. But he regained his spot later in the
2013–14 season and has been the first choice ever since. Arguably his best performance came in the early part of his career when he kept a clean sheet against Greek giants,
Panathanaikos. He repeated this feat the
following season as Panionios beat Panathanaikos 3–0 with Giannakopoulos as a main goalie. On 29 July 2015, Giannakopoulos signed his four (three years with an option for an additional one) years' contract with Greek giants
Panathinaikos for €180,000, adding another high-caliber Greek talent to their squad. On 17 April 2016, in the last game of the
2015–16 season, he made his debut with the club in a 6–1 home win against
Panthrakikos. As he is not among the first priorities of
Panathinaikos coach
Andrea Stramaccioni he signed a long-term year loan with
Cypriot club
Aris Limassol FC. On 26 June 2017, Giannakopoulos signed a two-year contract with
Kerkyra. On 4 July 2018, following the relegation of Kerkyra to the
Football League, he joined
Panetolikos on a two-year deal. On 13 March 2024, Giannakopoulos signed with newly promoted Finnish
Veikkausliiga club
Ekenäs IF. He was injured in his Veikkausliiga debut, in the opening game of the season. On 30 June 2024, his contract with EIF was terminated by mutual consent. ==Career statistics==