How can I add a GANTT chart to a board if there is none in the left menu?

Hello Laetitia,

by default, Gantt chart isn't enabled in a Board, unless your organization configures it differently. 

To enable the Gantt chart manually in a single board as a Board Owner, go to Settings:

Then, go to General.

As the last step, enable Gantt by clicking on this button.

Best regards,

Katarzyna Bobrowska