I don't like Gantt charts much, which you'll understand if you work with Agile Development methodologies like SCRUM.
But these kind of diagrams are still heavily used in lots of projects, so sooner or later you'll need a program to handle them.
There's the mother of all: MS Project.
What should I say, as many of MS Office programs, its heavy, slow, and error-phrone. And it's not free and doesn't run on Linux.
For Ubuntu there are several free applications available, and some of them can handle MS Project files, which is very important if your boss only handles this program.
Read my special article about this kind of applications, their dis-/advantages, special installation instructions aso.