I tend to find the visualization part of Cognos a bit wanting. They are getting better with Active Reports, but I suspect you're looking for a solution for live reports.
There are a few solutions.
1. SpotOn has a very nice looking Gantt solution.
http://www.spotonsystems.com/products/spoton-vantage/demos/gantt-chart-business-intelligence-demo/. I've never had the chance to use it, but it does look impressive.
2. FusionCharts has a Gantt chart available in their FusionWidgets line. It's a bit complicated to use it, as it involves some JavaScript, but it works well. I've written previously on how to embed FusionCharts into Cognos reports,
here,
here, and
here.
3. You could probably do something
nice cool acceptable using a crosstab and conditional formatting.