Guðmundur also played for the Icelandic national team for a decade, participating in two Olympics and two IHF World Handball Championship.

[3] In October 2013, it was announced that Guðmundur would take over as the head coach the Danish men's national handball team on 1 July 2014, replacing Ulrik Wilbek .

[4] On 21 August 2016 Guðmundur led his team to the gold medal at the 2016 Summer Olympics in Rio de Janeiro, Brazil.

[5] During the Olympics, Wilbek made two attempts to gain the consent of the Danish players to fire Guðmundur, once during the tournament and then the day after Denmark won the gold.

[13] In September 2021, he was sacked after 3 league matches only[14] On 21 February 2023, Guðmundur resigned as the manager of the Icelandic national team after five years at the helm.