Giannoulis was born on 5 June, 1976, in Toronto, Canada. Considered a great young talent at the center position, Giannoulis began playing youth clubbasketball in Greece, with the junior teams of Filathlitikos Karditsa and Ampelokipoi. After spending time with the youth teams of Esperos Kallitheas in 1992, he joined the junior team of PAOK Thessaloniki, where he played from 1992 to 1994. With PAOK, he won the Greek Under-18 championship, in 1994.
Professional career
Europe
Giannoulis began his pro career with the senior men's club of PAOK Thessaloniki. He was an All-EuroLeague Second Team member in the EuroLeague 2000–01 season, while playing with PAOK. In the next season, he won the EuroLeague 2001–02 championship, with Panathinaikos Athens. A failed drug test, at the end of the 2001-02 season, cost Giannoulis a ban from all European continental competitions for 2 years, which was a great blow to his career.
After playing professionally in the United States during his drug ban, Giannoulis returned to Europe to play in the Greek League, for the 2003–04 season. Giannoulis played with Panionios in the European-wide secondary level EuroCup competition's 2007–08 season, and had a very good individual season; and that same year, in the Greek League 2007–08 season, he helped to lead Panionios to a 3–2 series playoff victory over Maroussi, in the league's next season's 3rd place EuroLeague qualifier series. He then played with Panionios in the following EuroLeague 2008–09 season. In his club career, Giannoulis played with some of the following European teams: PAOK Thessaloniki, Panathinaikos Athens, Panionios, Málaga, Kyiv, Sevilla, Aris Thessaloniki, AEL Limassol, and Doukas. During his career, he competed at the highest European club level, the EuroLeague, in a total of eight seasons. In those eight seasons, he played in a total of 94 EuroLeague games, in which he averaged 6.7 points and 4.1 rebounds per game. Giannoulis played with the Cypriot League club AEL Limassol in the 2009–10 season. After that, he began a long career playing in the Greek minor leagues, starting with the Greek club Vouliagmeni, in the 2010–11 season.
Giannoulis was also a member of the senior men's Greek national team. He competed with Greece at the 1997 EuroBasket, the 1999 EuroBasket, and the 2001 EuroBasket. After being banned from continental competitions for two years, due to a failed drug test, he was removed from the senior Greek national team and he was never recalled to it after that. With Greece's senior national team, he had a total of 63 caps.