Select the Style tab from the Format pane on the right side of your sheet. The main thing to change is to make the blue bars transparent so that only the green ones remain visible. Now that your basic stacked bar graphic is created, you will need to make a few edits to turn it into a Gantt. Numbers will automatically generate a simple stacked bar chart that looks like the one below.
Select 2D Stacked Bar from the menu that appears.
In the second table below, you can see my calculations for the duration series. Use the next column to list each task's duration number of weeks required to complete that task.
I have included a second table in the image below to better illustrate how I calculated the start time series for my chart. This will help outline the span of the horizontal axis when you start building your graphic. In the second column, add the start week for each of the project phases, using the first task which will start in week 1 as the reference point for the rest of the tasks. Enter the key phases of your project called project tasks in the first column of the table as in the image below. In the newly added sheet, there will be a default table where you can input your data.