Gunilla died in the 16th century, roughly 500 years before, after which she was promptly taken to Valhalla, where she would eventually become the Captain of Odin's Valkyries and one of the thanes. At one point, she fell in love with a demigod son of Loki, who, however, ended up breaking her heart, as he turned out to be a spy sent by his father. As a result, Gunilla took to distrusting and hating all children of Loki.
Gunilla is first seen in the dining hall of Hotel Valhalla, where she questions Samirah al-Abbas about Magnus Chase's arrival. During the introduction ceremony, she is seen sitting with other thanes. Gunilla then plays a sabotaged tape of Magnus's death to purposely get Sam fired. Gunilla is seen after the trial, looking the smuggest.
Gunilla later gives Magnus a tour of the hotel and also shows him Asgard. She threatens to kill Magnus if he follows the plans of Loki and seeks to hasten Ragnarök. The next day, Gunilla sees Magnus trying to escape the hotel and says that she will kill him if he takes another step. Luckily, T.J. charges at her, giving Magnus time to escape with Blitz and Hearth.
She along with her fellow Valkyries and Einherjar (including Magnus's hall mates), travel to Midgard and see Magnus, Sam, Blitz and Hearth attempting to enter The World Tree through the "Make Way for Ducklings" statue. Luckily, with Hearth's magic and Sam's fighting skills, Magnus is given time to find the entrance to Nidavellir and along with his friends, escapes Gunilla, the Valkyries and Einherjar.
Gunilla later takes the form of a swan and arrives in Jotunheim to the castle of the giant Geirrod and his daughters Gjalp and Greip and is captured by them. However, Magnus, Sam, Blitz, and Hearth arrive and kill the Giants and save Gunilla. Despite being rescued by them, Gunilla (after returning to her human form) attempts to arrest the quartet and bring them to Valhalla. However, Gunilla is teleported back to Valhalla when Sam touches her Valkyrie armlet, though Sam believes Gunilla will come after them with reinforcements upon regaining full strength.
Gunilla along with two of her Valkyries and Einherjar, arrives on Lyngvi and sees Magnus with Fenris Wolf and believes that he is going to free the Wolf and tries to get the Einherjar to arrest Magnus and the others, but Magnus's hall mates refuse and side with Magnus.
Gunilla along with her two Valkyries are killed by the fire giant Surt. A funeral is later held for them.
Gunilla seems to be a very strict and militaristic person; she distrusts Sam, asking if Magnus was 'one of her father's spies'. She is also very serious, threatening Magnus that she would kill him if he betrayed Valhalla. Probably referring to Samirah as the two had been friends.
Gunilla is a tall, burly eighteen-year-old with pale blue eyes, and snow-blonde hair braided down her shoulders. According to Magnus, she is 'big enough to play power forward' and wears a bandolier of ball-peen hammers over her Valkyrie attire and a golden hammer pendant around her neck.
- Fighting Skills: As a daughter of Thor and Captain of Odin's Valkyries, Gunilla is an extremely skilled and formidable warrior, with roughly 5 centuries of combat experience. As a result, she managed to hold her own in a prolonged fight against Surt himself (along with her two Valkyrie lieutenants), but was ultimately defeated.
- Enhanced Strength - after gaining a new einherji body, Gunilla gained a degree of superhuman strength, to the point that she could punch through most walls without much effort.
- Einherjar Immortality - while within Hotel Valhalla, Gunilla, like any of the einherjar, could not die, and simply kept resurrecting from mortal wounds.
- Gunilla is a Swedish name, a variant of 'Gunhild', from the Old Norse name Gunnhildr, derived from the elements gunnr "war" and hildr "battle".
- Gunilla somewhat resembles Clarisse La Rue from the Camp Half-Blood series.