"Export Gathered Project" will only work if the project has "assets" to gather. These will be models, images and heightfields.
This will put all the objects, images and hightfields in a folder of your choice.
If your project has no assets you can save the project to a new location without using "Export Gathered Project".
Note that you will have to re-create any render output folders or re-map the output image location.
hope this helps
Richard