Jocelyn

A derivation from *Gautselin is inaccurate—the name was formed in the late Frankish period from Gois-/Gaus-, the Latinized and West Frankish derivative of Gaut in onomastic use (compare Cos- from Gaut- in Lombard names) and the Romance diminutive suffixes -el and -inus. It is then a Frankish/OF diminitive based on names such as Goisfridus (Geoffrey), Goisbertus, Goismerus etc., rather than an ancient Germanic name based directly on Gaut. Further Gauselin and the Gauselm (Germanic Gaut-helm) and their derivatives are frequently confused in the recorded texts, Goiselmus for Goiselinus and vice versa. [noted -ed]

Jocelyn is the medieval English name that means joy!

Could it be associated with the Gaelic descriptive surname Joyce/Seoige (from the Welsh word, "sais", meaning Saxon or English), with the diminutive -lin on the end?

According to the Dictionary of First Names, when Jocelyn entered English the spelling was changed because Joscelin was taken as a double diminuative (with the Old French suffixes '-el' and '-in') of Josce (Joyce).