One day members of his crew dared question his directions towards them. Gundar was enraged and attacked them, sword-in-hand. The whole crew was needed to face and defeat him. He fought filled with rage and hatred, a berserker rage. His sword broke and his strength abandoned him before the battle was over. But the defeated man cursed all his crew. They were to be cast on an unknown island ans spend eternity alone. Only Gundar could lift the curse.
Gundar abandoned the ship with only a small boat and vanished. That night the ship was caught in a storm and indeed shipwrecked. A god had heard Gundar's curse and fullfilled it. (The narration identifies said god as Odin. But the artwork depicts him wielding a hammer and controling thunder. He is then probably Thor. ) The crew members found themselves trapped and unaging.
In the 20th century, another sailor was shipwrecked in the island. He was able to lift the curse and send the Vikings back to their time. His name was also Gundar. He figured the elder Gundar was a distant ancestor of his, allowing him to lift the curse.