There is a similar issue with Ubuntu, where 19.04 (disco) is the latest version to include libgfortran3.
@marpet, would it be possible to include libgfortran3 in the download, or use libgfortran4 instead?
A similar report can be found here in this topic: Error due to missing libgfortran3, while libgfortran4 is installed (OpenSUSE Linux distribution) .