I figured out How to Add A Gantt Chart to Sharepoint 2010 Foundation last year, but I forgot and it took me over an hour to figure it out again, so this time I am writing the process down.  There are two ready ways to create a Gantt chart via the GUI:

CreateaGanttfromNewList:

The nut of the problem is that Microsoft calls this webpart a PROJECT TASK.

1. Click SITE ACTIONS (at top left of sharepoint window)
2. click MORE OPTIONS
3. scroll down and click on PROJECT TASKS

“…Project tasks lists provide a Gantt Chart view and can be openned by Microsoft Project or other compatible programs…”

How to Add A Gantt Chart to Sharepoint 2010 Foundation

Convert an Existing List to a Gantt Chart

1. Go to your list
2. Click the twisty from the the bread crumbs (at the top of the page)
3. Select CREATE VIEW
4. Select Gantt View

Create a Gantt Chart View For An Existing List in Sharepoint 2010