Create tasks and milestones
Aside from the start date and duration, every task may have priority, cost, color and fill pattern, text notes and user-defined custom fields.

Organize tasks in a work breakdown structure
Hierarchical tree where progress, dates or costs of lower level tasks is summarized on the higher levels. Summary tasks can be collapsed to hide tasks which are not important at the moment.

Draw dependency constraints between tasks, like "start X when Y finishes" and GanttProject will take care of enforcing these constraints. You can add a lag or use other types of constraints.

Create baselines to be able to compare current project state with previous plans.

PERT chart for read-only view can be generated from the Gantt chart.