GanttProject is a desktop application which works offline. You can work collaboratively the same way you do with Notepad or classic Word/Excel, by sharing files. There are many ways of doing that (e.g. you can send files by email).
We have our own collaboration service GanttProject Cloud which makes the task of sharing GanttProject documents easier. It is currently in beta, you’re welcome to try.
You can find some docs here: GanttProject Cloud Docs