Hi Aks26,
Unfortunately it is not posssible to hide the header details section of the Timeline header via the official API of the GanttChart component at this time. The header is designed to have two segments. I can only suggest that you to not set content to that section via the formentioned <b>timelineHeaderFormatFunction</b>.
Best Regards,
Christopher
Smart HTML Elements Team
https://www.htmlelements.com