Since you want to drag arbitrary elements from the outside into the gantt, there isn't a way for the gantt to contain this as a built-in feature. What I would advise is to use the Draggable UI widget. You can find its documentation here:
And here you can find live demos;
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.