Recent research however, shows that it is extremely unlikely, with three separate families of similar name having been 'grafted' together, the first join at about 1000 and again at about 1100.
The following is the original story as documented by John Hallum with the issues raised by recent research included as notes. John also noted that caution needed to be exercised over this early history.
Two such works of particular interest which came to light early in John's research are those published by a member of the family, John Henry Josselyn, namely;
"The Genealogical History of The Ancient Family of Josselyn of Horkesley" & "Genealogical Memoranda relating to the Family of Josselyn"
Both published privately at Ipswich in 1880 and 1903 respectively. (Refs 50 & 51)
Members of the "Josselin Society" are also researching the various strands of the family comprising the full range of spellings of the name and publishing their results amongst the Society members. Likewise, the internet also features considerably in disseminating original information and recent Josselyn research data.
Lodge's 'Peerage of Ireland' gives the account of the origin of family shown to the right.
The line of descent has been published in chart form by WF Joscelyne & AW Joscelyne (1984, revised 1996); their chart differs in detail from Lodge's account in showing Gerberga (Gerbuca) to be the niece of King Lothaire and daughter of Charles, Duke of Lorraine.
Gerberga (born 976) married Lambert I, 'The Bearded, Count Josceline of Brabant. The chart further goes on to trace the ancestry back through the Josceline male line a further thirteen generations to Duke Ydulf (6th century) and back through the female Gerberga line seven generations to Charlemagne (AD 742-814), which line is stated to be then further recorded back to Antenor, King of Cimmerians (Crimea), living 443 BC.
It is only to be expected that, with an ancestry spanning such a time span, there is inevitably a great deal of speculation and debate. Richard Joscelyne, (currently President of the Josselin Society), has recently published an article in the Society Journal questioning the existence of factual information or sources to back up the earliest claims for the family line prior to the middle of the 13th century at which point the family acquired Hyde Hall at Sawbridgeworth. (Ref 106)
With the proviso of the uncertainties of the last paragraph, this chapter is the first of several tracing the descent from the time of the Norman conquest to the family of Charles Josselyn, and other lines which branched off from his direct line, and which have latterly settled upon the Jocelyn or Josselyn spellings. (Those lines which have adopted other variations of the name will not be specifically treated.)
Godfrey, Duke of Brabant, having married Sophia, daughter of Emperor Henry IV, was father of Joceline, Lord of Petworth in Sussex and ancestor to the Dukes of Northumberland, whose male issue terminated in a Joceline, anno 1670.
But what undoubtedly proves the family to be of great antiquity is, that, when the Romans, after being Masters of Britain for 500 years, took their final farewellof it, and carried with them many of their brave old British soldiers who had served them in their wars both at home and abroad, they rewarded their services with a grant of Amorica, in France, which country included a great part of Normandy, and from them was afterwards called Brittany, and as those Britons gave their name to the country in general, so probably they did to the particular towns and seats they inhabited. It is, therefore, reasonably supposed that some of this family were among them, and gave their name to the town of Josselin, in the Department of Morbiham, in Brittany. But however this may be, the family derives its descent from Carolus Magnus (Charlemagne), King of France, with more certainty than the houses of Loraine or Guise, who so highly boast of it for:-
'The prime ancestor of our family married Gerbuca, daughter of Lotharius, King of France, descended from the said Carolus Magnus, and a younger brother of his descendant Godfrey, Duke of Brabant (ancestor of the Northumberland family), was the father of Egidius Josselin, a nobleman of Brittany, who passed into England for the Court of Edward the Confessor in the year 1042, and had issue Sir Gilbert Jocelyne'.
Charlemagne (Charles "the Great")|
(742 - 814)
King of the Franks 768
Emperor of the Holy Roman Empire 800
Emperor of the Byzantine Empire 812
|= Hildegarde of Vinzgau|
Louis I (Ludwig) "The Pious"|
(778 - 840)
King of Aquitaine 781
King of France 814
Ruler of the Holy Roman Empire 814
|= Judith of Bavaria|
Charles II "the Bald"|
(823 - 877)
King of West Franks in 843
King of Italy 875
Louis II "the Stammerer"|
(843 - 879)
King of France
|= Adelaide of Paris|
Charles III "The Simple"|
(879 - 929)
King of France, 893
|= Eadgifu (Edgiva) of England|
Louis IV "D'Outremer"|
(918 - 954)
King of France, 936
|= Gerberga of Saxony|
Charles I of Lorraine|
(953 - 994)
|= Bonne D'Ardennes|
Gerberga of Lorraine|
(ca. 975 - )
|= ca. 990, Lambert I "the Bearded" of Louvaine|
Lambert I "the bearded" of Louvaine
(c. 950 - 1015)
|married ca 990||
Gerberga of Lorraine
(ca. 975 - )
Lambert II "the belted" of Louvaine|
(ca. 991 - aft. 1062)
Egidius Jocelyn (G29)|
(ca. 1005 - )
Henry II of Brabant|
(ca. 1021 - aft. 1078/9)
Sir Gilbert Jocelyn (G28)|
(ca. 1040 - ca. 1131)
Godfrey I "Barbatus" de Louvaine|
(1060 - 1140)
(St Gilbert of Sempringham)
(ca. 1083 - 1189)
Egidius Josselyn (G27)|
(or Galfridus or Geoffry)
Joscelyn de Louvaine|
(ca. 1100 - 1180)
= Agnes de Percy
dau of William de Percy
(Jocelyn changed his name to Percy)
|Descent to Charles Josselyn|
Dukes of Northumberland|
|Josceline, 1670 (last male issue)|
Older sources such as Lodge (above) show the Josselyn and Percy family trees sharing a common ancestor, Lambert I of France, as in the diagram. However this would appear to be based on no greater evidence than them sharing a similar name.
Modern authoritative researchers such as Dr Katharine Keats-Rohan say that there is no evidence whatever, to show that Lambert I and Gerberer had a son Egidius (see for example, Genealogics - the website of Leo van de Pas - for a typical family tree).
|G28||Sir Gilbert Jocelyn|
|G27||Gilbert (St Gilbert of Sempringham)||Egidius (Geoffry)
Sir Gilbert Jocelyne, (G28) returned to France and later accompanied William the Conqueror in his expedition to England, where he was awarded possessions including the Lordships of and Tyrington (Torrington) , in Lincolnshire. He had two sons, Gilbert and Egidius (or Galfridus, Geoffry).
* Dr Katharine Keats-Rohan of Oxford University, the leading authority in the people of the Domesday Book.
Gilbert (G27), (St. Gilbert of Sempringham), the eldest son of Sir Gilbert, was born at Sempringham about 1082 and educated in France. He was physically deformed and thus not suitable to become a knight, so after his return he taught at a free school. He later became household clerk to Robert Blount, Bishop of Lincoln, and when that Bishop died, he stayed on at the court of his successor, Alexander. When his father died, he was presented with the parishes of Sempringham and Torrington, built on his father's land. Not being ordained he appointed a vicar but continued to lead a very strict 'monastic style' life, presenting most of the revenues to the poor. He was ordained a priest by Alexander and offered an archdeaconry but he refused the offer and returned to take over the parsonages of Sempringham and Torrington and also become Squire of the villages in 1132. He also established an order of nuns at Sempringham, the order continuing to grow such that he asked the Cistertians to take over the foundation. This they were unable to do but Gilbert was persuaded by Pope Euginius III to carry on by himself. The order grew to encompass some 1500 and became known as the Gilbertians, being the only medieval order of English origin and existing until the time of the dissolution, at which time there were some twenty-six monasteries.Gilbert himself became Master General of the order but resigned before his death due to loss of sight. He amazed others by his sparse living, particularly with regard to food, having at his table 'the plate of the poor' into which he put the best of his food for the benefit of the poor. He wore a hair shirt and took his rest by sitting in short spells, spending most of the night in prayer.
He assisted the Archbishop of Canterbury to escape from his pursuers and also backed Thomas a Beckett but escaped sentence with a pardon.
He died in 1189 aged 106 and was canonised by Pope Innocent III at Anagin in 1202. His relics were taken by Louis VIII to Toulouse and interred in the Church of St Sermin. The feast of St Gilbert is still celebrated on the anniversary of his death the 4th February.
The Parentage of St Gilbert is given in the Cotton Manuscripts;
"Pater ejus, Jocelinus nomine, miles strenuus, et vir bonus et opulentus, Neustigena natione, plures habens possessiones in partibus provinciae Lincolniae; mater veri ortu Anglica, a parentus fidelibus, non inferioris tamen conditionis originem trahens."
("His father, named Jocelinus, a strict soldier, and a good man and rich, of the Neustigen tribe, with several estates in Lincolnshire; mother of true English extraction of family servants but tracing her descent from no less worthy stock.").(Refs 55-60 & others).
Gilbert, after whom the Gilbertine order was named, was born to Jocelin, Norman lord of the manor of Sempringham in Lincolnshire, and his English wife in the early 12th century. He took the path of poverty, redistributing his goods to the poor and set up a religious house at Sempringham in 1131. Though it may not have been his original intention, a house of religious women grew up alongside, possibly out of a group of anchoresses. A second foundation at Haverholme quickly followed, though he himself gave up organization toward the end of his life, living as a hermit and wearing a hair shirt. He died at Sempringham in 1189 (Golding, Gilbert of Sempringham, passim).
|G28||Sir Gilbert Jocelyne|
|G27||Geoffry (Egidius) Jocelyn||= dau. of John Bissett|
|G26||Sir William Jocelyn||= Oswalda Goushold|
|G25||Robert Jocelyn||= dau. of John (or James) Fleming|
|G24||The Confirmed History
One possible candidate for father of Geoffrey was "Gozelin Filius Lanberti, Tenant-in-chief in Domesday Lincolnshire who held in 1086 lands previously held by his father Lambert. Probably a Norman. His son Gilbert had succeeded as lord of Redbourne by 1115/18." 
Lanbert, a Norman, occurs "Domesday Lincolnshire as tenant of Jocelyn fitz Lambert, to whom he was perhaps related. Perhaps the same as Lambert tenant of Odo of Bayeux." 
I don't know if Keats-Rohan plans to publish anything further on these families and I don't know of anyone else doing research on them.
The links beyond Geoffrey, though, ought to be considered unproven, though they were asserted as fact in the Joslin book by Edith Wessler . The purported linkage to Gerberga and to Lambert of Louvain all seems to stem from this source, and lacks evidence.
The pertinent references are:
1. French, Elizabeth, "Genealogical Research in England: (Joslin Family)," New England Historical and Genealogical Register, vol. 71, 1917, pp. 236-257.
2. Wessler, Edith S., Jocelyn-Joslin-Joslyn-Josselyn Family, The, Tokyo; reprinted, Higginson Books, Salem, Massachusetts, 1962.
3. Genealogies of the Stranahan, Josselyn, Fitch and Dow Families, privately published, Brooklyn, NY, 1868. New York Public Library call number: APV (Stranahan)
4. (probably Ralph de Insula), St. Gilbert, The Book of, Foreville, Raymonde, and Gillian Keir, Clarendon Press, Oxford, England, 1987.
5. Golding, Brian, Gilbert of Semperingham and the Gilbertine Order c. 1130-c.1300, Clarendon Press, Oxford, England, 1995.
6. Whittemore, Henry, History of the Sage and Slocum Families of England and America, 1908.
7. Keats-Rohan, K.S.B., Domesday People: A Prosopography of Persons Occurring in English Documents 1066-1166: I. Domesday Book, The Boydell Press, Woodbridge, 1999., 0 85115 722 X, 98-47670.