I’ve tried exporting my GANTT Project file to MSDPI (I need more functionality and so I’m trialling MS Project Online) but when I try to import the project into MS Project Online I get an error that 'That file isn’t an .mpp file and can’t be imported. Choose an .mpp file to continue.'
MPP is Microsoft’s proprietary binary format. GanttProject can only read MPP files, not write them. MSPDI is an XML based file format (it was invented by Microsoft either, but it is a text format, that can be read and written with less efforts). I suppose that Project Online wants MPP files, not MSPDI. You should contact Microsoft support and ask them how to import their own files into their own tool.