I think much of the Gesta is true if you strip away the half-human bear and the ogre-like fellow in Cornwall because the trip to Ireland seems to fit in well with the battles that were going on at the time.
There is some dispute about who his parents were. The Gesta names Leofric of Bourne nephew or grandson of Ralph the Staller (Constable to Edward the Confessor, 1011-1068) to and Aedina, great-great-granddaughter to Earl Oslac. Many authors have stated that Leofric is the Earl of Mercia and Aedina is Godiva of the naked horse ride fame, which makes Hereward a very important guy and explains why he was in charge of the defense of Ely instead of his (then) nephew Earl Morcar but I think Hereward was related to Earl Godwine instead. This flies in the face of what everyone else writes and genealogies are confusing and contradictory from the time but considering Ralph the Staller's age, I think it makes sense to make Leofric his nephew from an unnamed brother, step-brother, or half-brother. Ralph's mother was Goda, daughter of Ethelred II, and Dreux, Comte of Amiens, Mantes and Vexin. Ralph is said to have been born in Norfolk but his name is Flemish due to his continental connections. This makes Leofric (if he is the son of Ralph's brother) a grandson of Ethelred II(the Unready) as well and makes Hereward then possibly important enough to have his being outlawed by King Edward the Confessor necessary. If Leofric is Ralph's nephew, then he is also cousin to Ralph Guader, who fought on William the Conqueror's side at Hastings, making this Ralph second cousin or something to Hereward. Ralph Guader was rewarded with lands and became Earl of Norfolk.
Ralph The Staller is in some places married to Agatha or Gytha who is the sister of Godwine. If this is correct (big if), then Godwine's sister is Leofric's aunt by marriage, making King Harold a distant cousin to Hereward. The historian Anscombe wrote that Hereward was the OE version of Harold. Since names run in families, this may point to a familial relationship and makes Hereward first stop in Northumbria make sense.
Goda, Ralph the Staller's mother, also married Eustace II, Count of Bologne, and had children with him. Their daughter married Godwine's son. I have no names so this really needs to be investigated further but gives a further connection between Hereward and Harold. Eustace II is also important as a descendant of King Alfred the Great through his daughter Aelfthryth as he provides a familial connection to Gilbert of Ghent. It is so slim though but Gilbert, son of Ralph of Alost, is son of Adalbert of Gand, son of Arnulf of Holland son of Hildegarde, daughter of Arnulf I, son of Aelfthryth. I think one needs a big chart to work it all out but Gilbert's mother is Gisele of Luxembourg, sister to Ogiva who married Baldwin IV of Flanders. Gilbert (who is supposed to be Hereward's godfather) is nephew to Baldwin IV and cousin to Baldwin V, who Hereward was working for as a mercenary. Have I lost you yet?
They were all interrelated at this level but I am working on proving that Hereward was connected to the Earl of Wessex not the Earl of Mercia. Baldwin IV is father to Judith, who married Tostig, son of Godwine, who was Earl of Northumbria at the time Hereward was exiled. HaH! And there you go!
Back to Hereward's mother who was the great-great-granddaughter of Earl Oslac of York. There is not much information about his life but if he left family behind when he was exiled in 975, this could give Hereward another reason to go north after his banishment.
Due some details in the account of his trips to Flanders and some comments about when Hereward was spotted in Ireland and the fact that he was exiled at 18, I am going to declare him born 1040-1042. This is later than other people, who want him to be son of the Earl of Mercia and Godiva, but I am going with it. Then he was exiled in 1060.(since I favor 1042 as the year of birth)
In 1042 Hereward is born to Leofric of Bourne and Aedina. They have a large family. Hereward is a rascal and likes to attack people for fun. He goes after peasants who have not the weapons or the right to fight back. He also goes around the neighborhood taking rent or taxes from people that should go to his father, or possibly even taxes that should go to the king, and shares them out with his 'gang'. It could be the theft of the king's taxes that get him in the worst trouble and he was declared an outlaw in 1060, which means he has no legal rights under the law and anyone can kill him.
He heads north to Northumbria, possibly to obtain work with Tostig, who is back from his own exile and is Earl in Northumbria where he is dealing with disgruntled locals and raids from Scotland. I am going to go out on a limb and say that Gilbert was visiting Tostig who was married to his cousin Judith (their mothers are sisters). Since the Van Houts article also says that bears were trapped and shipped to the Continent for sport, it may be that a bear got out and Hereward dispatched it to the admiration of all and sundry but especially the women. There is no record of Gilbert visiting Northumbria so Hereward probably did not stay long. It is still in England and he is still and outlaw but Tostig could have told Hereward that his brother/half-brother Herbert (called by later sources FitzGodwin) in Cornwall is looking for mercenaries. Cornwall at this time is a part of the earldom of Wessex.
So Hereward travels to Cornwall, but finds work instead with the remains of the royal family of the former kingdom of Cornwall. I don't have a name for the princess but I suspect she is the sister of Cadoc (called Candorus or Cadocus by Camden). Following the Conquest, Cadoc is made Earl of Cornwall by William. He has one daughter, Alev, Alvive or Agnes (depending on who you read). She was married off to William FitzRobert, son of William's half brother. If the son of the King of Ireland had anything to do with her, this would have caused a big fight as there is land involved. She doesn't fit the timeline anyway but her name is close to Alef and so Leofric, the Deacon or Richard of Ely might have confused her for her grandfather, who was never named anywhere.
So Hereward spends some time at the court at Trematon and is sent to Ireland with a message for Cadoc's sister's suitor who is called a son of the King of Ireland. At that time, this would be Diarmait Mac Mail from Leinster who was High King at the time, but I suspect was Toirdelbach Ua Briain, who became later High King. He is the grandson of Brian Boru but his father was displaced from the throne as King of Munster by his half-brother, Toirdelbach's half uncle. In 1063, Diarmait supported Toirdelbach in defeating his uncle Donnchad for the Kingdom of Munster. The Gesta states that Hereward took part with the King of Ireland in an attack on Munster. I think this is the one but Hereward is said to have killed the Duke of Munster. Donnchad survived and went on a pilgrimage to Rome and so it may have been one of his sons who was killed. Toirdelbach was probably married three times and I don't know the names of his son's wives so I can't tell you who, if it really happened, ran off with a Princess of Cornwall who had already been given away to someone.
After Hereward went to Cornwall to rescue the maiden, he returned to Ireland with the prince and he obtained boats for himself and his men, who at this time include Siward the Blonde and Siward the Red, his kinsman who had been looking for him to tell him his father had died. From Ireland, he was going to head home but got caught in a storm and was shipwrecked on the coast of Flanders near Manasses the Old who was dead by 1065 so a shipwreck in 1064 or early in 1065 fits in with the timeline since Hereward presented himself to Manasses and possibly witnessed a charter for St. Sepulchre. In 1065 - 1066, Judith and Tostig are exiles in Flanders at St. Omer, which gives Hereward a reason to go to St. Omer where he meets Turfrida, his wife.
He is hired on as a mercenary and takes part in an action against Arnulf, the Vidame of Piquigny along with Baldwin II of Hainault and Lord of Brabant, Henry II, but he was trying to go under an alias and someone recognized him as the fellow in Ireland, three years before(1062-1063). Baldwin V (Gilbert's cousin) hired him to go on an expedition to collect taxes or tribute which some Frisian islanders have not been paying. Robert of Flanders (aka Robert the Frisian) is leading the venture. It is somewhat successful and Hereward picks up a horse in Walcheren. They return to Flanders, Robert's father has died (1067) and Baldwin VI is now Count of Flanders. His sister is Matilda who is married to William the Conqueror.
Gilbert, by this time, is in England as well as Eustace II, who both took part in the Norman side of the Conquest. At this point Hereward sees an opportunity to go home and have his exile lifted.
But while he is home, he fights some Normans who are living in his mother's home and have killed his brother. There no mention of where his mother is but his fight with and subsequent killing of the Normans eliminate his chances of not being outlawed anymore and Frederick, brother of William de Warenne, is trying to hunt him down. Hereward is forced to kill Frederick and then flees back to Flanders to wait for things to cool down. Flanders won't keep him for long since they are on the side of William so Hereward picks up his wife and the two Siwards and goes back to England. Being a part of the Earl of Wessex's clan and therefore related to Harold and having killed the brother of one of William's trusted advisors, Hereward has no hope now of being reinstated by William so he joins the resistance as it offers him a better chance, if it succeeds. He participated in a raid on Peterborough in 1070 and in the siege of Ely in 1071, where he escaped with a few men to the Bruneswald to carry on a guerilla war with the Normans.
He and Turfrida start to drift apart. Maybe she is tired of living in a ditch or maybe he has other women. She takes the veil. Hereward has been receiving offers of marriage from the wife of an Earl Dolfin, who promises that she can prevail over William and have him reinstated in his rights. The eldest son of a Gospatric, who was Earl of Northumbria at this time, was Dolfin who was abroad at the time. Gospatric took part in the rebellion of the north which William put down so severely. But if Dolfin was gone for a long time, and his wife did not want to be associated with his family anymore, she could have been looking for a new husband or William is likely to have found one for her. She wins anyway. Hereward married her and was reconciled to the king. He takes part in the 1073 campaign against Maine with William.
Ivo Taillebois, Ralph Malet ( who, as Sheriff of Suffolk, had a quarrel with Hereward's cousin Ralph Guader) and William de Warenne still want revenge for the stuff Hereward did while a rebel so they persuade William I to throw him into jail. While Hereward is being moved to Rockingham, Hereward's men attack and free him. Hereward spares his jailer, Robert of Horepol, who asks for clemency for Hereward and it was granted. Hereward is restored to his lands and lives out his days in peace with his trophy wife. His daughter by Turfrida, also named Turfrida married Hugh of Evermue.
So I am rejecting Gaimar's account of Hereward's spectacular death in battle in favor of the corroborating accounts from the Liber Eliensis and Croyland Abbey that he died in peace. I am also rejecting the suggestion he joined the Varangian Guard. If he was born in 1042, and he was in Maine in 1073 but was clearly dead by 1109, the earliest date given to the Gesta Herewardii, by 1100 he would have been 58, a hearty old age for the time. I am going to arbitrarily declare 1098 the year and he was 56 when he passed away. The end.