Gudni was born and brought up on a farm in SW Iceland 22. October 1958. After moving to Reykjavik in 1978 he finished BSc at the University of Iceland in 1983 and Cand. Scient in freshwater biology from the University of Oslo in 1985. He started working at the Institute of Freshwater Fisheries in Reykjavik as an assistant in 1979 and every summer to 1985. From 1985-2002 he worked as a fish biologist and a project leader at IFF and division leader from 2002-2016. From 2016 he has been a leader of the freshwater division of the Marine and Freshwater Research in Reykjavik. He has been a member of the ICES Working Group on Atlantic salmon since 1996. Member of the Scientific Advisory committee in NASCO 2003-2009. Member of the Icelandic delegation in NASCO 2008 and head of the Icelandic delegation in NASCO 2009. He was a Steering Group member of the CAFF, Circumpolar Biodiversity Diversity Program 2013-2017.
He has given lectures at the University of Iceland and the Agricultural Collage. He has been supervisor/co-supervisor to 4 masters students. His work has been on all three freshwater salmonid species in Iceland, and he has published 28 papers on Atlantic salmon and Arctic charr as well as over 250 technical and advisory reports.