On August 28, 2019, Kalaitzakis signed with Nevėžis Kėdainiai of the Lithuanian Basketball League, on a loan deal from Panathinaikos.

[9] Kalaitzakis made his debut in the NBA on October 19, 2021, coming-off from bench with two rebounds in a 127–104 win over the Brooklyn Nets.

[14] On July 25, 2022, Kalaitzakis signed a two-year contract with Panathinaikos, returning to Greece and reuniting with his twin brother Panos.

On December 7, 2023, Kalaitzakis signed a one-year contract with Tigers Tübingen of the German Basketball Bundesliga.

[16] On July 5, 2024, Kalaitzakis signed a one-year deal with Lietkabelis Panevėžys of the Lithuanian Basketball League (LKL) and the EuroCup.

[18] Kalaitzakis' twin brother Panos[19] is a professional basketball player and had been Georgios' teammate in both the Lithuanian club Nevėžis Kėdainiai as well as Panathinaikos.