I know of no record of a Hugh the Cupbearer but would agree with your statement that “Gitto” would not likely change to “Hugh.”
I don't see the purpose in inventing a modern surname (English Equivalent) for a person who lived in the time when place, function and honor names were used, centuries before surnames as we know them today came into use.The individual would not have used such a modern name and historians of earlier times would not have used such names.This practice could confuse future research engines causing real references in newly discovered ancient text to be overlooked and lost.
Based on the known time period for Hugh the Pincerna and William de Courcelles, Gitto must have lived before 960 and died sometime after 1055.This would put Gitto's birth in the period of king Bermudo II of the kingdom of Leon (who d. 999 and whose father was king Ordono III (d. 957).Bermundo II's wife was Elvira of Austurias.
Gitto's life would have paralleled the life of king Alphonso V ( who was b ABT 994, d. 5 May 1028).Alphonso V's father was Bermudo II of Leon (d. 999) and Alphonso V's wife was Elvira of Castile.
If the story is true then Gitto could have been a sibling of Alphonso V.Gitto was either a younger sibling or a deposed heir. The fact that he was permitted to use the Leon name means that, at least as a young man, he was entitled to it.He may have lived as a deposed heir under the protection of the Normans, been a younger sibling who was an official Leon liaison with the Normans, or a younger sibling soldier of fortune. The latter speculation is based on the Winston statement to the effect that Gitto had been a great soldier.
If this is true then Gitto's properties and position in Normandy may have been granted as a reward for some military exploit.Alternately, if this Gitto really existed and was not made up, he could have been someone simply from the kingdom of Leon not at all related to the royal family but mentioned in the context of his home place.It should be noted that all this is, of course, speculation.
Sources: Sir John Bernard Burke 1862 Chapter VII The Expansion of Europe, pages 193-196 and Table IX showing the bloodline of the kings of Leon and Castile
A History of the Middle Ages 284-1500 By Sidney Painter Published by Allfred Knopt