A Gantt developer designs and implements Gantt charts, which are visual tools used for project management. These charts illustrate the start and finish dates of the different sections of a project, enabling teams to easily track tasks and deadlines. The developer's responsibilities include coding the software to create these charts, ensuring that it can handle the input of complex project data, and making sure the output is easy to understand and use. They also test the software, fix bugs and may customize it to suit specific business requirements. In essence, they help streamline project management through effective visualization tools.