/usr/share/doc/libgtkglextmm-x11-1.2-dev/changelog.Debian.gz is in libgtkglextmm-x11-1.2-dev 1.2.0-8.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/libgtkglextmm-x11-1.2-dev/../libgtkglextmm-x11-1.2-0v5/changelog.Debian.gz