/usr/share/doc/libmgl-fltk7.0.0/changelog.Debian.gz is in libmgl-fltk7.0.0 2.1.3.1-4ubuntu3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/libmgl-fltk7.0.0/../libmgl7.0.0/changelog.Debian.gz