"How can I show a Gantt chart on my forms (or reports) using data in my database?"
Here's one solution that uses the built-in chart control. Gantt Chart using the Microsoft Chart Control.
If I wanted a sophisticated Gantt chart I'd use MS Project (hopefully I'd already be using MS Project if I was working with tasks and timelines.) But for a simple Gantt the built in chart control works great!
The example database shows the Gantt in a report but would work find in a form too, I think.