The words "truly historical" are mine, and as I've said elsewhere on this website, I am not a historian. I had previously described Gorm as the first King of Denmark, but this is clearly not the case as Denmark has apparently been a monarchy since the early 8th century. But Wikipedia refers to Gorm's father, Harthacnut, in several different places as "semi–legendary"; and Wikipedia's List of Monarchs of Denmark begins with Gorm (after about 17 "legendary kings" and three "semi–legendary kings" – the last of whom is Harthacnut).
Elsewhere, Wikipedia states that "The line of kings of the modern kingdom of Denmark can be traced back to Harthacnut father of Gorm the Old".
So (given that Harthacnut is semi–legendary) that is my justification for describing Gorm the Old as "the first truly historical king of Denmark".
© Haydn Thompson 2021