Gantter does not save projects as .mpp files, but you can export to a Microsoft Project XML file that can be opened using Microsoft Project.

Please use the "Export to MS Project (download)..." option in the "Project” menu of the Gantter editor to download the XML file.

If the issue is that you can't find the XML file from the Microsoft Project file picker, please make sure to select "XML Format (.xml)" or "All Files (.*)" from the file type drop-down. By default this shows "Projects (*.mpp)".

Please see the following link for additional instructions on opening the XML file in Project: https://docs.microsoft.com/en-us/office-project/xml-data-interchange/saving-and-opening-projects-in-xml-format?view=project-client-2016