This demo shows how you can use clickGraphItem event to register both click and double-click events on GANTT chart segments.
It works by delaying execution of click event using JavaScript function setTimeout. If the second click occurs on the same segment within 200 milliseconds, we cancel previous click and proceed as it was double-click.
P.S. the demo outputs information into browser's console. Press F12 and select "Console" tab to view it.