Guðmundur Steinn Gunnarsson's musical thinking is based on a rhythmical system that transcends the boundaries of pulse and traditional notation. This is often represented in his works through having musical symbols move on a computer screen, that is, animated notation.

His music has also been performed by the BBC Scottish Symphony Orchestra, the Iceland Symphony Orchestra, Ensemble Adapter, Apartment House, Riot, Crush, Caput, l'Arsenale, Mimitabu, Defun, Aksiom, ensemble.notabu, Elja, Nordic Affect, Ligeti Quartet, String Quartet Siggi, Reykjavík Chamber Orchestra, Duo Harpverk, Tøyen Fil og Klafferi as well as soloists such as Timo Kinnunen, Matthias Ziegler, Roberto Durante, Shayna Dunkelmann, Markus Hohti and Georgia Browne.

Guðmundur has lectured on his own work in Oxford, Princeton, Cornell, Huddersfield and Glasgow Universities and he was one of two keynote speakers at the ISSTC at Maynooth University in 2014. His work has been written about in articles, books and dissertations, most notably in The Digital Score by Craig Vear.

He has written over 250 works that have been performed in 25 countries. Guðmundur has been active in the composer collective S.L.Á.T.U.R. in Iceland and taken part in its festival Sláturtíð and the Jaðarber concert series as well as Fengjastrútur Ensemble. He has formed 3 ensembles for performing his own music, Fersteinn, Steinalda and USA Clangers respectively.