Hi Yaami,
Thanks for contacting Syncfusion support.
Currently there is no support to customize the weekend in Gantt control. We have already logged feature regarding this. A support incident has been created under your account to track the status of this requirement. Please log on to our support website to check for further updates.
https://www.syncfusion.com/account/login?ReturnUrl=/support/directtrac/incidents
Please let us know if you require further assistance on this.
Regards,
Mahalakshmi K.