Gutasaga (Gutasagan) is a saga treating the history of Gotland before its Christianization. It was recorded in the 13th century and survives in only a single manuscript, the Codex Holm. B 64, dating to ca. 1350, kept at the Swedish Royal Library in Stockholm together with the Gutalag, the legal code of Gotland. It was written in the Old Gutnish dialect of Old Norse.
The saga begins with Gotland being discovered by a man named Þieluar (Tjalve). He was a mythical figure who shows up twice in the Prose Edda and once in Gutasagan. Gotland is under a spell and under water during the day and out of water only during the night, a spell that is broken by Þieluar lighting a fire on the Island. Þieluar's son Hafþi (Havde) and his wife Vitastjerna had three sons, Graip, Gaut and Gunfjaun, the ancestors of the Gutar.
The saga says that after his father died, Gaut was appointed to be the chief, and shall have given his name to both the island and the Gutar people. They shared Gotland, where Gaut held the mid-section, Graip the northern and Gunnfjaun the southern part. Gotland was divided in three parts, a division that was reflected in a division of Gotland into three Tredingar, a division that remained legally to 1747 and still remains within the church, which still today retains this division into three Deaneries
That the Goths should have gone "to the land of the Greeks" is consistent with their first appearance in classical sources: Eusebius of Caesarea reported that they devastated "Macedonia, Greece, the Pontus, and Asia" in 263.
The emigration would have taken place in the 1st century AD, and loose contact with their homeland would have been maintained for another two centuries, the comment that the emigrant's language "still has something" in common shows awareness of dialectal separation. The events would have needed to be transmitted orally for almost a millennium before the text was written down.
The mention of the Dvina river is in good agreement with the Wielbark Culture. Historically, the Goths followed the Vistula, but during the Viking Age, the Dvina-Dniepr waterway succeeded the Vistula as the main trade route to Greece for the Gutar (or Gotar in standard Old Norse), and it is not surprising that it also replaced the Vistula in the migration traditions.
The Gutasaga contains several references to the relationship between Gotland and Sweden, and asserts that it is based on mutual agreements, and notes the duties and obligations of the Swedish King and Bishop in relationship to Gotland. It is therefore not only an effort to write down the history of Gotland, but also an effort to assert Gotland's independence from Sweden.
It gives Awair Strabain as the man who arranged the mutually beneficial agreement with the king of Sweden, and the event would have taken place before the end of the 9th century, when Wulfstan of Hedeby reported that the island was subject to the Swedes. (See Consolidation of Sweden)