We will push bug fixes and enhancements to the Odoo repository regularly. To get those fixes you can download the latest module version in a zip file from Odoo's app store page. In the module description is a list of latest changes.
You have to log in with the account that is used for the purchase of the module.
After download, add (unzip) the upgraded gantt module to the addon-path. Or, depending on your Odoo installation, merge the code in the zip file to your repository.
Then follow the steps provided in Odoo's upgrade manual
In some cases (fresh install) a restart of the Odoo server is required to include the module sources in memory of the Odoo instance. See the links below for corresponding command line options.