You use Gantt Chart for scheduling so it applies to all parts of the system development cycle from start to finish. You can make one say, for the different stages (eg. 3 days for understanding the problem, 3 days for making decisions, may overlap with designing solutions, etc..), and then go into more specific detail and create more Gantt charts within these stages. The point is however that different tasks can overlap and the Gantt chart allows you to show this.