In 1845 Gudmundur went to Copenhagen, Denmark, where he served an apprenticeship to learn the trade of goldsmith and watch making. He passed his exam as a goldsmith after four years of study. He worked in Copenhagen for another three years. It was during this time that Gudmundur heard of Mormonism. He attended a meeting where he heard the Apostle Erastus Snow preaching. He understood and believed the doctrine taught by Snow and his companions. He joined The Church of Jesus Christ of Latter-day Saints and was baptized 15 February 1851. Thorarinn Haflidason, a childhood friend of Gudmundur, was also studying in Copenhagen and joined The Church of Jesus Christ of Latter-day Saints. Three months later Gudmundur was ordained a teacher by Apostle Snow, and called to preach the gospel in his homeland. Thorarinn was ordained a priest, and he too was called to preach the gospel in Iceland.
In 1851 Gudmundur returned to his home in Rangarvalla, and Thorarinn went directly to the Westman Islands. After Gudmundur was rejected by his family and friends, he too went to the Westman Islands. Thorarinn drowned at sea in 1852. Gudmundur remained in Iceland about three years and baptized nine converts to The Church of Jesus Christ of Latter-day Saints. He then returned to Denmark, where he spent eighteen months doing missionary work.
Gudmundur wrote regarding his call to do missionary work in Iceland: “Having found the fruits of the Gospel more sweet and desirable than any other fruit, I expected that every person would believe my testimony, especially my own relatives. But alas, when I arrived in Iceland I preached to my brothers and sisters in vain. They would not believe me and, as my pious parents had died, I felt myself left alone, like Elijah of old, in the cave. However, I soon found a few believing friends who, not withstanding strong opposition on the part of the priests, were ready to embrace the gospel. I was often rebuked, spit upon and mocked by my enemies, but being full of love of God, I felt no anger or indignation against those who persecuted me.”
Gudmundur was arrested while preaching in Kalundborg, Denmark. After seven weeks in prison, he was released, but was then forcefully drafted into the army and sent to Copenhagen. His experience in the services was one of further difficulty, as his weakened health made training hard, and he was again subject to persecution and ridicule because of his beliefs. After a year, he was admitted into the hospital. He was eventually released and discharged from the army because of his poor health.
Gudmundur then made the journey to Utah, arriving in Salt Lake City on September 15, 1857. With him on the journey were many friends and families who had joined The Church of Jesus Christ of Latter-day Saints through Gudmundur's efforts in Denmark. Among this group was Nels Garff, his wife Marie, and their children. Nels' strength did not last through the difficult trip, and he died along the way. Honoring a promise made to him prior to his death, Gudmundur married Marie on 4 October 1857, and together they had three more children: Abraham, Isaac, and Jacob. Marie Jacobsen was born 9 December 1820 in Copenhagen, Denmark.
Gudmundur settled in Spanish Fork, as most of the Icelandic emigrants did; however, within two years he moved back to Salt Lake City. Gudmundur became involved with an apostate group of Scandinavian saints and joined a sect known as Morrisites. Gudmundur became a leader in that sect. He sold his home in Salt Lake City and moved to Farmington, Utah. In 1862 the Morrisites became fragmented and were disbanded. Many of the leaders were arrested, including Gudmundur. He was convicted of resisting an officer and fined one hundred dollars.
In 1868 Gudmundur took his family west to Sacramento, California to seek medical attention for Marie’s son Decan Westmorland, who had been ill most of his life. While Decan was recovering, Marie and Gudmundur had a lot of time to reflect on their lives. Marie wanted to return to Utah, where they could obtain a priesthood blessing for her ill son. It was during this time that their son Abraham and his friend found a large sum of money beneath the floor in an abandoned sawmill. Abraham’s share was reported to be worth about seven thousand dollars. Gudmundur reported this to the local authorities and with their blessing paid off their medical bills in Sacramento and returned to Utah.
Gudmundur and his family moved to Draper, Utah where they stayed for about two years. They then went to Lehi, Utah, where they purchased a home and Gudmundur set up a goldsmith and watchmaking business. Gudmundur was rebaptized into The Church of Jesus Christ of Latter-day Saints. Gudmundur died 21 September 1883 in Logan, Utah, where he was working temporarily. He is buried in the Draper, Utah Cemetery.