Jelle is a more recent form of Gelle (see the Submitted Names Database). The connection of Gelle (and ultimately Jelle) to Willem is also easier to understand if one takes into account that not only Wilhelmus was a common form of Willem in The Netherlands, but also Gulielmus (see the Submitted Names Database, also think of French Guillaume).Also, Jelle is a *short form*, not a diminutive, both for Willem/Gulielmus and Germanic names that contain the element 'gild.'
Also, Jelle is a *short form*, not a diminutive, both for Willem/Gulielmus and Germanic names that contain the element 'gild.'