Hi Oscar,
Thank you contacting Syncfusion support
There is no support to display milestones in Resource view Gantt, only tasks with valid durations can be displayed in Resource Gantt. In Project view Gantt, milestones are used to indicate an event but in Resource view Gantt, only valid tasks can be assigned to the resources and no milestones can be assigned to a resource. We suggest you to use strip line to indicate an even in Resource view Gantt.
Please check here for more details about strip lines in Gantt.
Please let us know if you require further assistance on this.
Regards,
Punniyamoorthi