Generally I like to start Gantt charts at the beginning of planning to make sure I capture my thoughts about how the project should move ahead. This may be nothing more than time-boxed phases that are loosely identified in the project charter. Or if I am in a consulting situation, I may provide a sample plan for the client to evaluate for my scheduling skills, to be sure the high level view of the project is realistic, and to create a document to plan around.
The Gantt chart should not be finalized however until after:
- the WBS is created and all estimates are completed (at least for the initial phases if you are going to practice succcessive elaboration or Agile)
- budget is near completion
- the risk assessment is completed (since risk responses may change thinking about planning)
Of course, I'm assuming this is a substantial project and warrants formal documentation in the plan for each project knowledge area.
In general, the project plan may not just be a single document, but a collection of documents that address each of the 9 knowledge areas for the project. The Gantt chart is a part of that collection and in small, informal projects it may be the only document.
At the end of planning, the Gantt chart should be the baseline by which the project is measured.