I don't think you will find much on Gitto.Historically the general belief is that the first Winston was incorrect in his identification.I have a descendency for the de Leons and Gitto is not among them; although, the pedigree does not extend to the children of the non-heirs.
From the Monasticon Anglicanum an entry from around 1100 A.D. reports a legal proceeding and says: "Hugh Pincerna witnessed a charter in favor of Saint-Amand, Rouen, Normandy before the conquest, and William de Courcelles his son, was the father of Roger de Courcelles."Crispin and Burke also echo this information.
This establishes that William's father was called Hugh the Pincerna.Indeed other members of the family both sons of Hugh and sons of William took up the profession of Pincerna, thus, the report seems confirmed.
Of course, Normans used various names by which they were known and their name could change in the various stages of their life.It is possible that Hugh was Gitto but there is no proof of that.Also, "de Leon" could have simply meant that Gitto was from Leon rather than Winston's interpretation which was that Gitto was of the Royal family of Leon.My understanding is that this same claim was made for Queen Elizabeth, if memory serves.