I use Gant as Portable APP because I can’t install anything on my PC. When I get to the window export, the export-assisten windows opens. I can choose the fomat. For example .pdf, than I get a window with other options as range, paper format, …but I can’t choose the botton OK (it turns from black into gray )…and I must abandon the assistent…
It means that the file where you’re trying to export to is not writable, or its folder is not writable or its folder does not exist. Provided that you use one of the latest versions of GanttProject, you should have seen corresponding error message under the file chooser at the top of this page.
However, as you mentioned portable app, chances are that you are not using the latest GanttProject, because, as to the best of my knowledge, portable package enthusiasts stuck with pretty old versions.