Short answers, pulled from the story.
Gunhild of Wenden is a Polish princess identified as the daughter of Mieszko I of Poland. Thietmar of Merseburg described her as the wife of Sweyn Forkbeard but did not record her name in his chronicle.
Adam of Bremen wrote his account decades after Thietmar's chronicle in the early 11th century. He claimed she became the mother of Olof Skötkonung through that union with Eric the Victorious of Sweden.
An inscription found in the Liber vitae of the New Minster and Hyde Abbey Winchester lists a sister named Santslaue. J. Steenstrup proposed that King Canute named his daughter after her mother, suggesting the Old Polish name Świętosława belongs to this unnamed wife of Sweyn.
Snorri Sturluson wrote Heimskringla during the 13th century collection of sagas where he described Sweyn Forkbeard being captured by Burislav king of Wenden. Negotiations required Sweyn to marry Gunhild while Burislav married Sweyn's sister Tyri according to the saga narrative.
Adam of Bremen's dual marriage report matches the Heimskringla account of Sigrid which suggests confusion between two wives or a duplicate memory of one historical figure. No source definitively proves which version reflects reality because primary evidence remains scarce and contradictory.