He studied at the Vocal and Acting Department of the Fryderyk Chopin University of Music, professor Alicja Dankowska-Prawdzic’s class. He had his debut in the role of Szóstak in S. Moniuszko’s Flis staged in Polish National Opera. Szostak took part in semi-finals of Belvedere Vocal Contest in Vienna. From 1993 till 2003 he was a member of the Artistic Company of Polish Armed Forces. From 2003 he was a soloist of Teatr Muzyczny in Lublin and since the beginning of 2008/2009 season Grzegorz Szostak has worked as a soloist in the Grand Theater Łódź. He has got in his repertoire the following roles: Amonasro in G. Verdi’s Aida, Don Basilio in G. Rossini’s The Barber of Seville, Zbigniew in S. Moniuszko’s The Haunted Manor, Crespel and Luther in J. Offenbach’s Tales of Hoffmann, Tevye in G. Bock’s Fiddler on the Roof. He gave performances in the Grand Theater Łódź in roles such as Sarastro in The Magic Flute, Caspar in C. M. von Weber’s Der Freischütz, Alfred Doolittle in F. Loewe’s My Fair Lady, Vodník in A. Dvořak’s Rusalka. He worked with many directors, like H. Kupfer, A. Leistenschneider, W. Zawodziński, A. Strzelecki, B. Augustyniak, W. Ochman, J. Gruza, J. Niesobska, Z. Czeski, A. Rozhin, M. Korwin, T. Cyz, M. Treliński and conductors: J. Maksymiuk, T. Kozłowski, J. Boniecki, A. Knap, M. Nowakowski, T. Bugaj, G. Chmura, P. Sułkowski, Ł. Borowicz, T. Karolak and T. Tokarczyk.
Grzegorz Szostak collaborates with Polish National Opera in Warsaw, Baltic Opera in Gdańsk, the Castle Opera in Szczecin, the Mazovian Music Theatre in Warsaw, Teatr Muzyczny in Łódź, Warsaw Philharmonic, Zabrze Philharmonic, Płock Symphonic Orchestra, Brass Band of Świętokrzyska Philharmonic and Victoria Brass Band.