Short answers, pulled from the story.
Gorm the Old emerged from a semi-legendary lineage in the early 10th century to become King of Denmark. He ascended the throne after his father Harthacnut died and likely ruled Jutland initially before expanding his authority.
One theory places Gorm the Old death in the winter of 958 or 959 based on wood felled in 958 used for the burial chamber at Jelling. This dating comes from archaeological evidence found inside the northern mound where he was buried.
Gorm the Old commissioned the oldest of the Jelling stones in honor of his wife Thyra to mark her status as Denmark's Adornment. This monument stands today as a national symbol of Denmark despite its ancient origins and reflects the high status of his queen.
Saxo Grammaticus later claimed Thyra played a role in constructing the Dannevirke fortification but excavations show the oldest phase dates back to 500 AD or earlier. Further expansions occurred much later during Harald Bluetooth's reign long after Thyra died though some historians suggest she may have influenced one section.
Gorm the Old had three sons together named Toke, Knut and Harald. Harald would eventually become King Harald Bluetooth after his father's death and left the original hill as a memorial after moving the remains.