By default, when GanttProject Desktop opens a document from GP Cloud, it will not write it to the local disk. Project data are stored in memory and in the cloud.
However, sometimes you may want to keep a local offline copy, e.g. if you’re going to work on your project while not connected to the Internet. There is an option in GanttProject Desktop which makes a local offline copy of a cloud document.
When you are connected, the offline copy updates as you save your work and is synced with the cloud copy. When you go offline, you can still edit your projects, updating the offline copy. When you go online, it will try to sync with the cloud copy.