Gudmunsson competed in the men's 100 m breaststroke at the 2000 Summer Olympics in Sydney. He achieved a FINA B-cut of 1:04.57 from the Mare Nostrum meet in Barcelona, Spain. He challenged seven other swimmers in heat four, including two-time Olympians Valērijs Kalmikovs of Latvia and Arsenio López of Puerto Rico. Entering the race with a fastest-seeded time, he faded down the stretch to touch the wall for a sixth spot in 1:05.55, a 1.5-second deficit from joint leaders Kalmikovs and Lopez (a matching time of 1:04.02). Gudmundsson placed fifty-second overall on the first day of prelims.