So you're saying that the technicality of whether or not he actually at any time ruled a hotly disputed territory, Vestfold, causes you to doubt that he was the son of Gudrød Bjørnsson ?
Or are you saying that a petty king of Vestfold could not have come from a line of Norwegian kings?
Kings ruling some or all of Vestfold:
Halfdan the Black, 9th century king of Vestfold. His brother was long believed to be buried at the Gokstad Mound.
Erik Agnarsson
Halfdan Hvitbeinn (part of Vestfold)
Eystein Halfdansson
Halfdan the Mild
Gudrød the Hunter
Halfdan the Black, together with his brother, Olaf Gudrødsson
Ragnvald the Mountain-High, Cousin of Harold Fairhair
Harald Fairhair
Bjørn Farmann
Olaf Haraldsson Geirstadalf, brother of Bjørn
Harald Gudrødsson Grenske, 976–987
I haven't found any sources disputing his rule of Vestfold, although he is described as a "petty" king. And obviously they fought with the Danes over it for a long time.
When the sons of Gunnhild had been banished, Harald Grenske followed Haakon Sigurdsson who ruled Norway as a vassal of the Danish king Harald Bluetooth. Harald became the king of Vestfold and Agder. He married Åsta, the daughter of Gudbrand Kula.
Harald Grenske (10th century) was a petty king in Vestfold in Norway.
His widow, Åsta, promptly gave birth to a son, Olaf Haraldsson, later Olaf II of Norway and patron saint of the nation. Åsta subsequently remarried to Sigurd Syr, the king of Ringerike.
Åsta Gudbrandsdatter (c. 975/980 – c. 1020/1030) was the mother of two Norwegian kings, King Olaf II of Norway and King Harald III of Norway.
Contemporary historical genealogists apparently agree that King St. Olaf was the son of Grenske.
https://no.wikipedia.org/wiki/Olav_den_hellige