Thank you for your answer Mr. Abdoul, but my question is the default group by product list is still being shown even there is no records at selected date period at Odoo Default Rental Module and mine doesn't.

Custom module, did you create it from scratch or did you customize the location module?

I developed the custom module and use the Odoo Gantt View at my custom module, but default group by - product list is not still sticky when there is no records at selected date periods but Odoo Default Rental Module does

I see, this is probably because it is a custom module. It is up to you to define the default grouping of the Gantt view, unlike Odoo Rental, which has been predefined by Odoo itself.

Thanks for your reply Mr. Abdoul, I still can't find the method how Odoo does that sticky default group by product list.