/usr/share/doc/libqgis-gui2.18.17/changelog.Debian.gz is in libqgis-gui2.18.17 2.18.17+dfsg-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/libqgis-gui2.18.17/../libqgis-core2.18.17/changelog.Debian.gz