You can configure your Gantt chart to show parent items either as actual tasks or as groups - containers for tasks. A group aggregates values from the tasks it contains and its own values for manual start/finish dates or work estimation are ignored.
If you want your parent tasks to be shows as actual tasks, you need to remove the "Allow parent issues to be treated as groups" checkbox. Otherwise they will be treated as groups.
If a parent issue contains non-issue items only (folders or Confluence pages), it will be shown as a task regardless of the selected option.
This option is applied to the whole Gantt chart and can not be configured for individual parent issues.