Update: So SQL Server 2008 is here, and still no Gantt Chart. So this free native workaround will still get you through on SQL Server Reporting Services 2008.
I've got an article over at SQL Server Central that describes how to get Gantt charts in SSRS using native controls, not third party. And it has a holiday spin.
This is the abstact:
Gantt charts in SSRS aren’t included in the 2005 release. You can purchase a third-party add-on, or, if you can wait, these powerful project tools might make it in the 2008 release. Or Alternatively, you can do this now with the native RS Chart control by using the methods David Leibowitz provides with the native RS Chart control.
This is the URL: http://www.sqlservercentral.com/articles/Reporting+Services/Santa+gets+an+RDL+present/61782/
The RDL example file is found in the Attachment link below.