Durham United He served as an unused substitute keeper for
Durham United FC for a match in
League1 Ontario in 2014, but did not play.
Ocala Stampede In 2015, he played for the
Ocala Stampede in the
Premier Development League.
Devonport City In March 2016, Giantsopoulos joined
Devonport City in the
NPL Tasmania, the second tier of soccer in
Australia.
Launceston City Giantsopoulos returned to the Tasmania regional division, signing with NPL Tasmania club
Launceston City for the
2018 season. He was named vice-captain of the club, and started all 21 matches in 2018. On August 18, in a match against
South Hobart, as a penalty shot was about to be taken, Giantsopoulos ran up and kicked the ball away before it could be taken. He was yellow carded, but his antics proved to be effective, as the ensuing penalty shot was missed. The incident became a viral video around the world. This was part of a streak of three consecutive penalty saves in three matches for Giantsopoulos. He was named to the NPL Tasmania Team of the Year at the end of the season. He served as the backup goalkeeper behind
Marco Carducci. He made his debut on May 16 in a
Canadian Championship match against
Pacific FC. After the season, he re-signed with the club for an additional year. After two years with Cavalry, Giantsopoulos left after the 2020 season in an attempt to gain more playing time. In his limited playing time for Cavalry, Giantsopoulos earned four clean sheets and had the best save percentage of any CPL goalkeeper.
York United FC On November 10, 2020, Giantsopoulos signed a two-year contract with his hometown club,
York United. He made his debut for York United on July 7, 2021 in a 2-1 victory over
Valour FC. On May 20, 2022, immediately after earning a clean sheet against
Pacific FC, Giantsopoulos joined
Major League Soccer club
Vancouver Whitecaps FC on loan for their match on May 22, as the Whitecaps were left without any of their three goalkeepers due to injury and COVID protocols. He did not feature in the match, serving as an unused substitute. His loan expired on May 23, allowing him to return to play for York in their
Canadian Championship match on May 24, where he made the game-winning save in the penalty shootout to advance to the semi-finals, where coincidentally, York was matched against Vancouver. In December 2022, he re-signed with York United on a one-year extension for the 2023 season. He began serving as team captain during the 2023 season. After the 2023 season, he turned down a contract extension offer from York, and he began training with his former club Cavalry, but did not ultimately sign. He later joined
Vancouver FC during their 2024 pre-season.
Vancouver FC In April 2024, Giantsopoulos signed with
Vancouver FC for the 2024 season, with an option for 2025. Despite not making any appearances in the 2024 season serving as the backup keeper, Giantsopolous signed an extension for the 2025 season, after the 2024 season. In March 2025, he announced his retirement. ==Coaching career==