Currently there is no support for load on demand as like your requirement, but it has built-in support to render the nodes multi-level hierarchies in virtualization approach by using “EnableVirtualization” property. In virtualization mode the Gantt control renders or we can able to visualize only few nodes of data at a time, depending upon the height of the Treegrid control (viewport/view height), thereby increasing the responsiveness in Gantt while scrolling, expanding, collapsing and other such actions. The child records won’t get render until we expand its parent as per your requirement. In virtualization concept, the scrollbar or scroller of Gantt is intended to render as if there are large data, but actually the rows which are viewable alone are rendered, even while scrolling the Gantt content. You can achieve the virtualization in Gantt with below code snippet,
<ej:Gantt ID="Gantt" runat="server"
We have also prepared a sample based on this and please find the sample under the following location.
We have also logged a feature request regarding “Support for load on Demand for Gantt” this. It will be implemented and available in any of our upcoming main release.