Hi Marcel,
I have consulted the development team in this regard, and understand that this feature has not been implemented. We have prioritized this request (feature# 297) and will try our very best to implement the same at the earliest. The progress of this feature request can be tracked at this link below :
Feature Request #297 - Support for a splitter in a form hosting a GridBagLayout manager
We appreciate your patience and thanks for your continued interest in Syncfusion products.
Regards,
Guru Patwal
Syncfusion, Inc.