This is an update to my previous post on Gantt chart web part by Teun. WSS 3.0 version of this web part can be found here. Although MOSS has an out of the box Gantt view for lists as can be seen below the SharePoint Gantt view is bloated compared to the Gantt chart web part. Not only do you get the nice rounded corners for the chart but it will pick up Group by settings of the tasks list and display a black container around the grouped tasks.
Many people have reported problems activating this web part on the site, to resolve this try running the following 2 stsadm commands after you have used the install scripts, these commands will run timer jobs and activate the features on your site collection.
The web part should appear under 'miscellaneous' section in the gallery with the name 'Gantt chart'
stsadm -o execadmsvcjobs
stsadm -o activatefeature -name Duynstee.GanttChart -url http://<site url>