A group aggregates values from the tasks it contains, and its own values for manual start date, finish date or work estimation are ignored. You can configure your Gantt chart to show parent issues either as actual tasks or as groups (containers for tasks).
Structure.Gantt groups issues by default. To have your parent issues shown as actual tasks, on the Scheduling screen, under Groups, uncheck the "Treat parent issues as groups" checkbox.
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.
Grouping preference is applied to the whole Gantt chart and cannot be configured for individual parent issues.