- TaskJuggler: excellent free software on task scheduling and report generation, with beautiful GUI and powerful command-line interface.
- Bugzilla:TaskJugglerIntegration: export Bugzilla tasks to TaskJuggler project file.
The script bztotj.py from Bugzilla:TaskJugglerIntegration is easy to understand, and by changing the SQL in it you can export tasks from Bugzilla by arbitrary filters.
TaskJuggler's task is written in plain text so it's good for change management. It also features a nice GUI editor. You may need some time to learn to write project files but once you get it you can write versatile project files and, by writing scripts to manipulate them, implement complex project scheduling schema.
If you want to generate Gantt from TaskJuggler tasks without human interference, you can use this tool: http://blog.gustavobarbieri.com.br/2007/04/19/generating-gantt-graphs-from-taskjuggler-projects-without-using-its-ui/