Note: https://en.wikipedia.org/wiki/Turstin_FitzRolf ? but is much later.
https://www.geni.com/people/Hrolf-Bigod/6000000009437210537
From The Conqueror and his companions, Volume 2 By James Robinson Planché (paraphrased):
Rongwald (Ragnvald) had several children by a favorite slave whom he had married "in more Danico", and one of these was Hrolf Turstain, who followed his Uncle Rollo into Normandy and secured the hand there of Gerlotte de Blois, daughter of Count Thibaut de Blois et Chartres. Hrolf was "the foundation of this branch of the great Norse family in Normandy, and the stock from which descended the Lords of Briquebec, of Bec-Crispin, of Montfort-sur-Risle, and others who figure as companions of the Conqueror."
The third son of Hrolf Turstain and Gerlotte was Ansfrid the Dane, the first vicomte of the Hiemois (aka Exmes). His son Ansfrid II (called le Goz) was father of Thurstan (Turstin) le Goz, a trusted aide to Duke Robert of Normandy who accompanied him to the Holy Land and was entrusted to bring back to the Abbey of Cerisi the relics the Duke had obtained from the Patriarch of Jerusalem.
Don't think he was the Ganger's son, more likely his nephew. Apparently his father was Ganger Rolf's evil twin half-brother.
See Also * http://www.geni.com/people/Rollo-Thurstan-Bigot/6000000009437210537
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E > Eysteinsson > Turstin FitzRolf Thurstan Eysteinsson
Categories: EuroAristo - Profiles that need work | European Aristocrats, Unsourced Profiles
"This house can be distinctly traced back to the father of the first Duke of Normandy, Rognavald, Earl of More. Besides his two legitimate sons, he had, by a favourite slave whom he espoused more danico, a third, named Hrollager, who settled with them in Neustria. Hrollager's three grandsons each became the founder of an illustrious Norman stock. From the eldest, Anslac de Bastembourg, came the Bertrams, Sires de Briquebec, and the younger house of Montfort-sur-Rille; from the second William, the barons of Bec-Crespin; and from the third, Ansfrid the Dane who was Viscount of Exmes, or Hiesmes, before 978, the house of Avranches. He was the first Viscount of Hiesmes that is on record, and his descendants inherited this dignity, as well as his surname of Le Gotz or Gois. Toustain Le Gois, his grandson, was Chamberlain to Duke Robert the Magnificent, Stood high in his favour, and went with him to the Holy Land; but having rebelled against his successor, forfeited the whole of his possessions, which were granted to the new Duke's mother, Arletta. Toustain's son Richard, however, who had never swerved from his allegiance, obtained his pardon, and set matters straight by a judicious alliance. He married Emma, or Emmeline, de Conteville, Arletta's daughter, who brought him back all the lands that his father had lost; and acquired numerous other estates, notably in the Avranchin, from whence he took his name. In Duke William's charter to the Abbey of St. Evroult (about 1064), he signs himself Richard d'Avranches, being at that time Seigneur or Viscount of the Avranchin."
So according to the above, Rognavald (Earl of More), was Father of Rollo (First Duke of Normandy), thus the Averenges or Avranches descended from them?